The major reason why learning Chinese isn't like learning Japanese is the same reason why learning Chinese isn't like learning any other language:
Chinese doesn't have grammar. Period. I love the language, it's my mother's mother tongue, but it just doesn't have any grammar, which is why it's so difficult to learn for people who haven't been brought up with it to some degree; if you were brought up speaking anything else, you're used to there being rules for how words work. Those rules don't apply in Chinese. No rules do. You've got an adjective? Hey, it's a verb now. And still an adjective. And it might even be a noun. All in the same word. All in the same sentence. I'm glad I understand how it works, but I have no idea whatever how to try explaining it.
Japanese, on the other hand, does have grammar. It has grammar hard-style. The primary reason, I think, why westerners have a hard time with Japanese is that its grammar is (a) non-Indo-European, and (b) makes so much more sense than any Indo-European grammar I've ever been exposed to. Okay, so I haven't been exposed to Esperanto, but since that was made by Indo-European speakers, I can only assume that it makes even more sense than that.
I have a talent for languages, and so my story for learning Japanese reads something like this:
Oh, hey, there's this internet thing here. Oh, wait, there's this Japanese hentai on this internet thing here. Well that stuff's kinda weird. Huh. But I do sorta wanna know what they're saying. I'll get a Japanese dictionary and a simple Japanese grammar book.
Oh, wait, that didn't actually help at all.
Oh, hey, there's this anime-stuff over here. I think I'll watch some and become a Japan-obsessed nerd. Doo-de-doo-de-doo... Okay, I think I'll watch it in Japanese now. And try to translate the songs every now and then.
A few years later...
Holy crap I understand what these people are actually saying, not just random individual words. Well, that came completely out of nowhere. Huh. I guess I'll use this skill to better the world. (-starts translating hentai-)