For Sci-Fi, my settings tend to be all-encompassing; anything goes. They way I look at sci-fi is that it's a big galaxy (sometime the scope ranges into the universal spectrum), so a place can be found for just about anything I want to use from any source. In fantasy settings, the same thing can be done, but my mindset is different for fantasy. My problem, however, is that I've got so much material that I would love to use that it's difficult to keep myself focused on a smaller scope. I'm trying to do that with the Mystara hack I'm working on now. (I hesitate even calling this setting Mystara since I'm changing the setting a lot.) Still, even with the idea of D&D iconic places, which is going to increase the size and scope quite a bit, I find myself looking for a place for Green Ronin's Freeport, Sword and Sorcery's Hallowfaust, and other bits and pieces from other company's settings that I think are cool and would like to use. It's part and parcel with having such a large collection of material to draw from. The collector in me buys up anything interesting, the creator in me dreams up of all sorts of ways to apply the new material, but the realist and writer in me sees a mountain of work to bring it all to reality. I'm sure it's a problem that a lot of gamers have, so I'm likely not stating anything that many of us haven't encountered time and time and time again.
One of the ways that I'm going to cut down on the scope is limiting the playable races. I tend to allow players to select a wide array of races. While I'll offer more than standard (much more so), I'm not going to allow a lot of the more "montrous" races like Orcs, Gnolls, and the like. Goblins will still be allowed, but they are going to be more akin to Goblins like those seen in the Iron Kingdoms setting (marginally socially accepted travelers that roam in gypsy-like familial groups). What I'm trying to go for is a sort of a "points of light" style of setting. There are kingdoms and such, and trade between cultures, but travel is dangerous and the wilderness is filled with danger; sort of the Civilized Vs. Uncivilized world in which the uncivilized are winning the war. This will be primarily an exploration and adventure campaign rather than much political intrigue or political hoo-hah in general.
I keep telling myself that one of these days I'll just take a small adventure and build the world from there like I did back when I first started out with the Keep on the Borderlands. Have the setting come to fruition one adventure at a time...then I look at all the books and pdf's in my collection, many of which have never seen use, but are chock full of great ideas and my will to let a setting flow from the adventures naturally falters pretty quickly.