The in-play zone is now the battlefield.
This was the single biggest Oracle change. A lot of cards—no, seriously, a lot of cards—do something when they enter the battlefield, or do something when they leave the battlefield, or enter the battlefield with counters on them, or put a token onto the battlefield, or so on. It stands to reason; the battlefield is the primary game zone. A total of 2,417 cards now say "battlefield" in their Oracle wordings.
There are a number of variations in the wordings. "Put [something] into play" became "Put [something] onto the battlefield." "When [something] comes into play" became "When [something] enters the battlefield." "From play" and "in play" became "from the battlefield" and "on the battlefield," respectively. There are others, but you get the idea.
As part and parcel of this change, sometimes other phrases within a card's wording got juggled around so they'd sound better. Despite this, no functional changes were incurred as a result of this change. All the cards are the same; they just go to a more interestingly named place.
The removed-from-the-game zone is now the exile zone.
The exile zone is a shorter, more accurate name for a zone that was, increasingly, part of the game. And, as a consequence, the instruction to "remove [something] from the game" is now an instruction to "exile [something]." If something referred to the "removed card," it now refers to the "exiled card."
This was another big change, covering 675 cards. Two sets of cards functionally changed as part of this revision: the Wishes, and the imprint cards. The rest work like they always did.
The Wishes didn't change because they got a new template; they changed because the rules changed. Cunning Wish, for example, lets you get an instant card you own from "outside the game." This used to include the removed-from-the-game zone, even though it isn't really outside the game (just ask a suspended Greater Gargadon). With its new name, the zone is obviously inside the game, so a Wish can't get a card from there anymore.
Imprint cards pretty much work like they always did. The only thing that changed is that you can no longer do tricks involving cards like Cytoshape (such as changing a Duplicant into a Mirror Golem, and doing something different involving the "imprinted card"). Each imprint card has an ability that exiles some number of cards, and one that refers to the exiled cards. The second ability now refers only to cards exiled with the first. The cards were already worded in such a way to make those tricks nearly impossible, but now all loopholes have been closed. There's a little more information about imprint in the Comprehensive Rules section of this article.
"Play" is now "cast," "activate," or "play," as appropriate.
For the past ten years, you played a spell, played an activated ability, played a land, and played a card. Not anymore. Now you cast a spell, activate an activated ability, and play a land. However, depending on the context, you may be instructed to cast a card or to play a card.
To cast a card is to cast that card as a spell. This wording is used when the card can only be cast as a spell, and can't be played as a land. For example, Knowledge Exploitation tells you to search your opponent's library for an instant or a sorcery card, then says that you may cast that card.
To play a card is to cast that card as a spell or to play that card as a land, whichever is appropriate. For example, Knacksaw Clique has an opponent exile the top card of his or her library, then says that you may play that card.
Some cards still use "play" when it seems like they should use "cast." For example, Spelljack counters a spell, exiles it, then tells you that you can play the exiled card. This may seem strange—since the exiled card was a spell before, it seems like "cast" is the appropriate word. The reason "play" is used is because Zoetic Cavern exists. As printed, if Spelljack counters a Zoetic Cavern cast as a face-down spell, it lets you play the exiled Zoetic Cavern card (which is a land). So it will continue to do so. Zoetic Cavern and Dryad Arbor (and, to a lesser extent, Ancient Den and its artifact land friends) threw some monkey wrenches into the play/cast/activate divisions. The intent, of course, is to incur no functional changes. Everything should work just as it did before ...
... with one bizarre (and practically impossible) exception. There's a small group of cards that have "enters the battlefield" triggered abilities, but they're supposed to give you the benefit only if you cast the card as a spell (and not if you Zombify it, for example). Coal Stoker used to say "When Coal Stoker comes into play, if you played it from your hand, add to your mana pool." Now it'll say "When Coal Stoker enters the battlefield, if you cast it from your hand, add to your mana pool." Seems fine. The difference is if you manage to play Coal Stoker as a land—the old way would give you , the new way won't. How do you play Coal Stoker as a land? Simple: Have a Coal Stoker already on the battlefield, equip it with Runed Stalactite so it's a Saproling, control Life and Limb so it's also a land, and play Vesuva copying it. Sadly, you'll no longer get any mana in this situation. Just thought you should know.
1,335 cards now say "cast" in Oracle. (This includes a number of uses in reminder text.) 388 cards now say "activate," but that includes cards that already said that (such as "Counter target activated ability").
Abilities that triggered "at end of turn" now trigger "at the beginning of the end step."
Abilities that trigger "at end of turn" have been confusing players for years, since they don't actually trigger at the true end of the turn—they trigger at the beginning of the end step, which is the second-to-last step of the turn. After that, players can still cast instants and activate abilities, then the player whose turn it is discards down to seven cards, then damage is removed from creatures and "until end of turn" effects end. That's a long way off from the end of the turn! Now such abilities will trigger "at the beginning of the end step," which is both more accurate and more in line with "at the beginning of your upkeep" triggers.
The most confusing cards in this vein were the ones that created a delayed triggered ability that triggered "at end of turn." For example, Rakdos Guildmage's second ability was ": Put a 2/1 red Goblin creature token with haste into play. Remove it from the game at end of turn." If you activate this ability during the end step, it's too late for the delayed "at end of turn" ability to trigger that turn, even though it seems like it isn't. These abilities get the same fix, except they also use the word "next." Rakdos Guildmage's second ability now says ": Put a 2/1 red Goblin creature token with haste onto the battlefield. Exile it at the beginning of the next end step." (We didn't come to this conclusion in time for Magic 2010 cards, so a couple of them are missing this "next.")
As part of this change, what used to be known as the "end of turn step" is now the "end step," and the "end phase" is now the "ending phase." A couple of cards reference these, so they've also been changed accordingly. In all, 250 cards now reference the end step (many just in reminder text).
Finally, a number of cards were printed with the reminder text "This effect doesn't end at end of turn." This was meant to imply that the effect has no duration; it lasts for the rest of the game (or until the affected permanent goes away). These now say "This effect lasts indefinitely" in Oracle.
Effects that last "as long as" a condition is met now last "for as long as" that condition is met.
There are two kinds of "as long as" abilities. One was an independent on/off switch (basically, the "as long as" could be replaced by "if"). For example, "Ashenmoor Cohort gets +1/+1 as long as you control another black creature." These weren't changed, though in some cases, the "as long as" clause was moved to the front of the ability for clarity.
In the other kind, "as long as" represents a duration that's set up by an earlier part of the same ability or by the ability's activation cost. That kind of "as long as" effect is true from the moment its ability goes on the stack or starts applying until the "as long as" condition stops being true. For example, Aven Mimeomancer's "At the beginning of your upkeep, you may put a feather counter on target creature. If you do, that creature is 3/1 and has flying as long as it has a feather counter on it." All 62 of these changed to say "for as long as" (except for Orcish Squatters, which said "for as long as" all along). Some got sentences rejiggered in the process so they'd sound better.
Nothing functionally changed here. It's a small addition, but it should help make these kinds of abilities clearer.
Lifelink has changed.
As has been previously written about (and is discussed again in the Comprehensive Rules section), lifelink is functionally changing. The 10 cards printed with lifelink reminder text are getting new reminder text in Oracle. The 16 cards printed with the ability "Whenever [this creature] deals damage, you gain that much life" that got errata to have lifelink are being reverted to their printed abilities. (The one exception is Loxodon Warhammer, which has been printed both ways. It was most recently printed with lifelink, so it's keeping that functionality.)
Deathtouch has changed.
Like lifelink, it's functionally changing. The 15 cards printed with deathtouch reminder text are getting new reminder text in Oracle. The two cards (Cruel Deceiver, Venomous Fangs) printed with the ability "Whenever [this creature] deals damage to a creature, destroy that creature" that got errata to have deathtouch are being reverted to their printed abilities.
Reminder text is helpful.
Sometimes, if a block-specific keyword like transmute or suspend has long reminder text, we'll leave that reminder text off of some cards (usually rares) for space reasons. Grozoth, for example, has transmute but no reminder text. When you're busting open Ravnica packs, the odds are that if you've found a Grozoth, you've also got some other transmute cards that explain how the ability works.
A few years down the line, however, that doesn't necessarily hold true. It's much more likely for a player to pick up a random Grozoth without any other Ravnica cards nearby and have absolutely no idea what transmute means. If that player looks up Grozoth in Gatherer, there's still no help. He'd have to do a general transmute search, or dig up the Ravnica FAQ, or (heaven help him) look in the Comprehensive Rules. We can be more helpful than that.
All such missing reminder text has been added to Oracle for the sole purpose of being there in Gatherer when someone looks up a card. Reminder text for phasing, banding (I apologize in advance), horsemanship, and fear has also been added.
In addition, if a card had trample reminder text in Oracle (because it was printed in a core set, most likely), that reminder text has been updated to the M10 reminder text (which references planeswalkers).
This covers 253 cards in all.
Colorless tokens are colorless.
If a spell or ability makes a colored token, it always specifies what color that token is. If a spell or ability makes a colorless token, it specified that the token is colorless only if that token wasn't an artifact. In other words, something that made an artifact creature token made you figure out that it was colorless just by not naming a color.
This is actually fine by the rules. Tokens have only the characteristic values specified for them. If no color is named, the token has no color. But this was a bit of an inconsistent policy, and it wasn't particularly clear (especially with Shards of Alara block cards that made colored artifact creature tokens). The 20 cards that didn't specify that they make colorless tokens will now do so.
Clone has a new template.
Clone has a new, streamlined wording in the M10 set: "You may have Clone enter the battlefield as a copy of any creature on the battlefield." Accordingly, the seven other cards that make use of this wording also will.
Mana burn no longer exists.
Six cards (Upwelling, Mindslaver, and four Kamigawa block Snakes) mentioned mana burn and/or emptying mana pools in their text. Since the rules changed (there is no mana burn, but now mana pools empty at the end of each step and phase, not just at the end of each phase), these changed accordingly.
Substance no longer exists.
Twelve cards (Waylay, Thawing Glaciers, and the ten-card Armor of Thorns cycle) had "at end of turn" triggers back when that truly referred to the very end of the turn. To preserve their functionality (especially because some of them were practically nonsensical if they were sacrificed before damage clears), they were given a wording that allowed their delayed triggered abilities to trigger during the cleanup step (a time when nothing normally triggers). To achieve this, the relevant permanents gained substance (a static ability with no effect) until end of turn, then triggered when they lost substance—because "until end of turn" effects wear off at the same time that damage does.
These cards will now have a more straightforward "at the beginning of the next cleanup step" trigger, and the rules have been adjusted accordingly. Note that this new wording is somewhat misleading, since it's not the first thing that happens in the cleanup step (just like "at the beginning of your draw step" triggers aren't the first thing that happen during your draw step—you draw a card first). In the cleanup step, the player whose turn it is will discard down to seven cards, then "until end of turn" effects will end and damage will be cleared, then these abilities will trigger.
Assassin's Blade and friends haven't been accounting for planeswalkers.
A number of combat tricks from the Portal sets were printed with the restriction "Play [this card] only after you're attacked, before you declare [blockers/interceptors]." They had the Oracle wording "Play [this card] only during the declare attackers step and only if you are the defending player." But these two wordings aren't the same, because planeswalkers exist.
If one or more creatures attack your planeswalkers, but none attack you, then you're still the defending player—but you haven't been attacked. According to the printed wordings of these cards, you shouldn't be able to cast them. Thus, they're changing to say "Cast [this card] only during the declare attackers step and only if you've been attacked this step." This affects 14 instants and the activated ability of Kongming's Contraptions. (Blessed Reversal dodges this change because it was reprinted in a few other sets with a different wording.)
Cards that referred to the top cards of graveyards didn't all say "of."
Most cards that refer to the top of something (either a graveyard or a library) use "of": "Look at the top three cards of your library, then put them back in any order," for example. Some cards that dealt with the graveyard didn't follow suit, because they adhered to other conventions (like "returning" the top card "from" your graveyard to the battlefield). All cards that deal with the top of something will now use "of" in Oracle. This affected six cards.
I pulled this from the Wizardsofthecoast.com. I site them as my source.