You can have sword-and-sandal tales of scantily-clad barbarians versus crypt-lurking sorcerers, you can have willowy elves and furry-toed hobbits contending with goblins and trolls, you can have Klunk the Gnome flinging alchemist's fire from the deck of his steampunky airship... heck with a little tweaking you could conceivably run a fairly realistic historical game of Napoleonic warfare if that's your bag.
Most D&D worlds are set up to allow any or all of these things to happen, sometimes all at once, which makes for a completely nonsensical setting, but does at least cast the widest net for player preferences. Rare is the group where everyone wants to play Sharpe's Rifles, but give that one player the Pathfinder Gunslinger class and a natty uniform, and he'll probably be content trotting along next to Bonk the Barbarian and the catfolk cleric.
This is a good thing, when it comes to pleasing a group of players. And heck, sometimes that sort of an "anything goes" world can actually be very entertaining. But my problem as a GM is that I am a compulsive worldbuilder and tend towards fantasy snobbery if I don't catch myself, so I start geeking out on all the fiddly bits of the world and trying to make it all work in context and make sense and not come falling down like a house of cards when you start thinking about the ramifications of cure disease spells and wondering why there are castles when a single wizard is way more dangerous than an army and and and...
To get around this, I often try to pick a strong, well-established "theme" for any given game I run, so even if the tropes don't make sense, they are at least a package deal that's known to "work" together. Thus it is I've run the "Middle-earth clone" game and the "Greyhawk dungeon-crawly" game and so on. Right now I'm in the midst of an Eberron game, which is "extrapolate a setting from the artifacts of D&D rather than impose D&D on a setting."
But my other problem is that I get impatient and fickle. Yesterday I downloaded a demo of a game called Loren Amazon Princess, which is just about the most fanservicey, fan-ficcey, generic fantasy cliché game you could ever find. Seriously, this game is like it's 1995 and I'm playing Quest For Glory again, except with a PG-13 rating. I enjoyed it quite a bit, even while wincing at it (I suppose it's a strong example of a "guilty pleasure"), but it also made me miss my very first D&D game, back in high school. (Strictly speaking, that game was not D&D, it was a more-or-less nonsensical homebrew system, but it might as well have been.)
That game was full of all kinds of dorky stuff like you might expect from a high school D&D game-- "red smurfs" as monsters, warrior princesses hanging around in dungeons waiting to be found by adventuring parties so they could join the group, player characters with names like "Slick Rick" and "Zarfbardafardwards." I don't know if I could maintain a game like that, I suspect I couldn't, but dang if it wouldn't be fun to play a session or two. (I don't think I could stand doing it with Pathfinder, either. The system has too much overhead, too many rules, too many moving pieces of math that need to work together. Savage Worlds might be able to make it work.)
But this is what I mean when I say impatient and fickle. There's no reason I couldn't toss some of that stuff into the Eberron game if I really wanted to, other than that it's been pretty "iz srs bzness" so far. Maybe I should throw some nonsensical stuff into the next dungeon just to break myself out of the habit of thinking too much. I did want Eberron to be freewheeling and fun after all.