These days, I'm big on internal consistency. That doesn't mean that it has to be
realistic -- this is fantasy role-playing, after all. But the game world has to be
believable or else my immersion in the game is ruined. Many game worlds are created with absolutely no thought to issues such as ecology, or the effects that widespread magic use and magical healing would have on the population in general.
A unique flavor to the world is also important.
I used to love the old D&D "Known World" (now Mystara). It was the first campaign world I ever played in (and DMed). It was a lot of fun, with a lot of detail, but when I try to think logically about how all of the different bits fit together, it makes my head hurt. All of those small nations within such a small area, but with such differing cultures -- Romans next to Arab/Egypt next to Vikings next to Mongols -- is a bit too much to swallow. Never mind the deserts bordering on fjords. The world also has a fair bit of just plain sillyness about it.
I've always found Greyhawk to be very bland and boring. It suffers from the unbelievable ecology issue, but not as bad as....
The Realms. What can I say that hasn't been said already?

I used to really like the setting, back in the days of the 1987 Grey Box. If I could erase my memory of any FR materials published after 1988, I might be able to enjoy the setting. But it is over-developed, and the over-commercialization of the FR in the early 90s really killed it. There are entirely too many extremely powerful predators roaming the land (what do they eat?), 22-level dungeons built for no real purpose other than to amuse an insane mage, powerful wizards in every village, etc, etc. The world just strains my suspension of disbelief too much.
Dark Sun was one of the better official D&D worlds. It's a bit too unconventional for my tastes, but it was a good campaign setting.
Birthright was probably my favorite TSR-published world. It had a level of internal consistency that I hadn't seen in many published worlds, and the ecology of the setting was much more believable. It would have made a great campaign setting even for a regular style of D&D game, where the players aren't rulers.
Kingdoms of Kalamar seems to possess all of the traits that I like in a world, from what I have seen. But every time I pick up the book to consider buying it, the naming conventions turn me off. I can't, with a straight face, tell my players about a Fhokki warrior who just returned from Svhimozia (sp?). Many of the names are hard to pronounce, impossible to spell, and have far too many apostrophes. It's too bad, because the setting seems very consistent and believable.
Looks like my next game will be homebrew again. But I just picked up the Book of the Righteous, and I'm seriously considering building a world around the mythology in the book; it's that good.