I've actually be having a lot of fun thinking about this and getting ready to move into planning and design.
First and foremost, I'm going to use this as a learning exercise to teach myself some new technology, so don't expect a very rapid turn around. I figure I'll either build the site using Ruby on Rails or Django, so that'll be cool. I think I have the option of using PostreSQL on the backend, so I might do that, as well. (Not that I expect this to be a big test of a database...)
As for features, I think we want the following for submissions: tagging, commenting, searching, ranking, and maybe a "favorites" pool (favorites could work as a binary ranking system... the more a submission is "favorited", the more popular it is...). Ideally there would be an RSS/Atom feed for the latest submissions.
Users need profiles with optional links to blogs, Flickr accounts, MySpace pages, etc.. They could also have some semi-social functionality like "buddies", and so forth.
Hopefully we can run out some t-shirts/stickers/whatev from the best submissions. There's some vague potential for some actual goods, but that's nowhere near reality at this point.
What else do we want/need?