It was tough choice for me between Dragonlance, Planescape, and Forgotten Relams, but I had to go with Realms in the end.
1. Too many dag-blasted powerful NPCs (esp Drizzt and Elminster) who too many DMs use to show up and save the day at the end.
2. Too much magic- its pretty bad when every 1st level FR adventure I have played in has the characters getting multiple permanent magic items. Its a powergamers paradise!
3. The lands, cultures, and geography make no sense- its too much of a melting pot with no thought as to how all of these wildly divergent places came about in the first place.
4. The horrible novels that inundated the market a few years ago. For all its faults, FR was still mostly playable if the DM re-worked it heavily, but with the novels, too many players took them as canon, and if a DM didn't incorporate everything in those books, players got offended, or told the DM he was running the Realms wrong! Of course the DM could tell his players he wasn't running the Realms according to the books, but I have seen that attitude make some players VERY angry, to the point of walking out of the game and/or intentionally causing trouble during the session.
Dragonlance is also a big offender to me as well- mostly for reasons other people pointed out (too scripted, only one major background story in the world, many players too familiar with the setting), but also because of the over-emphasis on dragons. With dragons being so common, they lose a lot of their charm and mystique.
Planescape never appealed to me either, mostly because it seemed overly factionized (too much like a WW game), and because of the silly slang terms and DiTerlizzi's artwork had completely the wrong feel for it to me.
Ok, enough ranting- my favorite published D&D settings were probably Greyhawk (lots of fond memories playing there when I was younger, and the world seemed bigger, more complicated, and more mysterious than the Realms) and Birthright (it had interesting cultures, the world made sense, and it gave the PCs the chance to do something they rarely did in other games- run a small barony and have to be responsible!). Kalamar looks pretty cool and well thought-out, but I never have had a chance to play/run a game set there yet.
