-Interesting concept. If you have the same array of kingdoms, barbarian tribes, evil races, gods, etc. just with new geography and different names, don't even bother. I'd rather just play one of the several existing vanilla settings that don't require learning all those new names and geography. Instead, I want something to be different about the setting, a general concept or group of concepts that shapes the flavor of the setting and makes it a different experience than any other setting out there.
-Tweaked and/or new races. Tying into the above point, I don't want races to be exactly the same as the standard. The tweaks need not be mechanical in nature, but I like some thought put into how the races have been affected by or grown out of whatever is different about the world. After all, if the world is different than anything else out there, it would certainly shape its inhabitants in different ways.
-Variety of fun ideas. When I read the setting, I want to constantly be thinking "wow, that's a cool idea/place/npc/whatever, I should use that in an adventure/make a character that ties into it."
By contrast, I care less about believability, consistency, and level of detail. To a certain degree, things should make sense, but if there is a choice between keeping things exactly consistent and believable or having an intriguing backstory and a colorful backdrop to adventures, I'm quite willing to suspend disbelief. By the same token, I'm pretty comfortable coming up with standard cities/kingdoms/whatever on the fly, so I'd rather have nuggets of really unusual or interesting customs, locations, and so on than pages and pages of mundane geographical description.