There's a few details you need to make clear:
A) Is this for D&D or d20 Fantasy? (readies a glare for diaglo) D&D has its own themes that are seperate from the basic rules that it uses (dungeon-delving, etc).
B) How much are you willing to tweak mechanics?
C) What's more important, the world, or the adventures?
Now, as for my personal tastes:
1) Keep the most basic fantasy, ignore Greyhawk entirely. That is, swords, magic, etc, all good. I hate guns (their effect on history annoys me, despite my being a happy gun owner), so, take that as you will. Melee weapons and the supernatural are always good.
2) Everything else you've come across is optional. Don't feel constrained by what has come before.
3) Avoid basing things on the real world as much as possible. Asia is fun, the Middle Ages of Europe are fun, but you're going for unique, not ripped-off. Being inspired is one thing (Like my 'black' cannibal culture... who have the highest number of paladins of any culture in my world; they cannibalize out of respect and love -- grandpa would rather be jerky than undead.), but don't try to get away with a Maztica.
4) Avoid basing things on other fantasy worlds. Pretend there is no Tolkein. Make up your own fantasy critters (then again, so long as you have something noticably supernatural, you don't even need fantastic creatures for fantasy, but they're so darn fun...) as much as possible, even if they're just variations on an archtype. A creature that starts a joke and makes you finish it else it eats you, with an ape's front and a hippo's back, with a pair of moth wings attatched, is an obvious sphinx reference, but good for a giggle at the least. I, myself, use massive serpents instead of dragons.
5) Make it make sense. I can't stress this enough. Virisimilitude is a -powerful- device in fiction. If your reader, accepting the rules of magic and such, can actually think, "Yeah, that COULD happen!", you get serious points. This is part of why Planescape and Dark Sun were so beloved: they took a messed-up situation and actually made you go "Yeah, that makes sense."
6) Deus ex machina is disgusting. Now, if you honestly, truly, have a battle of the deities occur and mess up some mountains, and it actually makes sense for mountains to have been messed up by two guys smacking each other around, fine, do it. But deus ex machina explanations are weak and eye-roll-inducing.
7) This is the Information Age. People today like information. Never forget that.