Well, in my case it'd be that someone else actually wanted to DM...but that doesn't really answer your question.
A generic fantasy world is open, permissive and fun with lots of room for PC creativity. But it can get out of hand. I've heard the arguments against permissive settings before and I just don't buy it. Say you DO actually have a guy who really wants to play a ninja. There's no way you can tell me that there's absolutely no strange-little-corner-of-the-world that you could make up for him to be from, and make it somewhat believable. (this assumes a reasonably "normal" D&D world.)
I'll go against the grain here and say that I don't believe that setting is key. I enjoyed the Star Wars movies in spite of the fact that I didn't know anything about the planets that Chewie, Han Solo, or any of those guys came from. (We only knew that there was some kind of life-oath that Chewie owed Han) We didn't even know much about Tatooine except that it had 2 suns and was nothing but a big frigging desert. It just wasn't integral to the story that was being told. The setting was just a backdrop.
There's a pattern in fantasy fiction, and if you read a lot of it, you know it. The first couple pages of the book include a map of the entire realm/continent. You can be pretty much assured that all the major points on the map will see some sort of action before the whole cycle of books is finished. These stories are often like a grand tour of a whole continent. Many guys (unfortunately, IMO) model their campaigns the same way. The whole continent is laid out and every flavor detail of the setting set down, with no wiggle room to account for offbeat characters. As a player, I often find this sort of inflexibility unfortunate.
Having said that, the DM does a lot of work, so I don't begrudge it that much and will usually play along merrily...I'm just happy to be playing for a change. (an aside: I really only get annoyed when the DM is a total control freak and begins overruling even the PHB on a regular basis for the sake of his setting. Then I'd usually walk.)
You can run a whole campaign within a specified region, where what lies outside that region is only known in the vaguest of terms. One reason I quit using FR was that it was so well known by my players that even characters born and raised in Cormyr seemed to have an almost intimate knowledge of all the other major spots in Faerun, from Waterdeep to Thay...which really stretched my own sense of believability. I persanally think of common fantasy world folk as having, at best, even the foggiest notion of what lay beyond the next mountain range, let alone the rest of the continent. Anything they DO know, came from storytellers.
My point here is that, to the perspective of that average commoner, there's lots of strange lands beyond that which I know, and it isn't too far a stretch to say that new guy in town with all these strange abilities is from one of those places that I don't know. Pretty much leaves my setting open to almost anything.
There's a saying that used to get thrown around a lot in the old TSR days. I believe it was the title of a regular column in Dragon. The phrase is "The play's the thing". I don't know how it was meant back then, but I take it to mean that game considerations trump all else. A great setting is nice, but not integral to a good game. You can have a wonderfully fleshed out setting, with every rock and tree itemized and every civilization detailed, but still run a stinker of a game. Believe me...I've seen this from both sides of the screen.
A good game needs a story, a villain, goals, and a compelling reason for the adventurers to adventure. It doesn't much matter if your home base is a village built high in the tops of ancient Redwood trees just of the verge of a tundra, or a village that hangs suspended between colossal sized palm trees on a desert continent somewhere. Either way, its a village in the trees where you go to sleep at night, and a place where you may have to stand and be counted when the time comes to defend it.
And, maybe they have an extra room for that strange ninja guy from the land of Far Away that you've been hanging out with lately.
