But sometimes you just need a village, or an island, or a tavern, or you want some ideas for a distant race, or you need a good evil cult quick.
Indeed - but notice that most of the things you mentioned as needing are on a micro level. An individual location in a city such as a rowdy tavern is easy to use and integrate - an entire nation is not. The headquarters of an evil cult mapped and described down to encounter level is quite useful - an entire evil religion and god is a lot more difficult to integrate.
Why? Well, gods, races and nations rarely exist in a vacuum, and they're relatively easy for DMs to create based on putting some themes together. Low level detail, such as the tavern and cult HQ that PCs can walk into - that's worldbuilding stuff I can see a use for, ala the locations in Magic of Faerun and Lords of Darkness.
So, if you go for the worldbuilding option, I'd say that an emphasis on micro level detail is probably the go. Macro level detail would - I assume - be more difficult to integrate and map to a homebrew world except in the planning and building stages, because by virtue of being big and wishy-washy, it can tend to step on the toes of existing material.
Unsurprisingly, most of the worldbuilding you see is at macro level, and if you ask for submissions, that's probably what you'll get. Worldbuilders are rather fond of putting a dot on a map with a city name beside it, and, if you're lucky, writing a paragraph of material about the feel or politics of the city in broad strokes. They then move on to a mountain range etc. Just look at the size of the city books for how much detail can go into a single city.
If you don't mind just gathering ideas, then it should be fine, but you can get that from existing settings. If you're doling out components, you can either go for a few paragraphs of ideas on a lost race, or a mini-adventure or description of a hidden lair where the PCs encounter said lost race. I know which is more useful to me as a DM, and if the site is touted as worldbuilding, I expect you'll get a vast majority of the former rather than the latter. Worldbuilders love painting in broad strokes, and as long as you're happy with that kind of material then there's no problem. I just think that under this kind of model, you'll find very little crunchy stuff - think of the netbooks - and therefore, IMO, very little reason to visit the site.
Ideas are a dime a dozen, whereas detailed locations, NPCs and mini-encounters aren't - and often contain ideas implicitly.
6 cents.
