Romance anime, when done well, is probably my favorite genre of anime. Unfortunately finding a good romance is extremely rare.
I wonder sometimes; somewhere along the way did the people who write these plots just stop caring? Take Ah! My Goddess for example. The first few episodes were focused on their developing romance and their unique situation. The show had potential! Unfortunately it goes downhill once Skuld arrives. From that moment on the anime shifts focus away from romantic comedy and warps into something more like a sitcom. Pretty much every single romantic moment they have is interupted by skuld, marller attacks, or urd screws something up.
Mamotte Shugogetten is another example of this, the first episode had potential, then suddenly 4-5 other extremely annoying characters come into play. There's like three guys who constantly break into his house and try to propose to Shao. They literally consume every moment from there onward.
Another thing I find annoying is anime like Love Hina and Girls Bravo. Why take the wimpiest most mediocre guys imaginable, and place them in a situation where they're continuously beat up by the girls around them. Is that comedy? Did I miss the punchline or something? Imagine if the main character was a woman, and they were all men beating on her. Not very funny.
The basic recipe for a good romance is to keep the character cast simple! The plot can be complex and weird, as long as you give the two main characters some alone time and don't fill the show up with annoying side characters. Follow that and you have the makings of a successful romance. Onegai Twins, Shakagan no Shana, and Midori no Hibi are good examples of this. All of them have very unique plots.