There are SourceForge and Savannah for free software; Wikimedia for free knowledge - but no repository exclusively dedicated to free art.
- music, including tracker music, classical compositions etc.
- sound files (samples)
- drawings, paintings of any kind
- web comics (fully remixable!)
- artistic photography (i.e. no general photo archive - there's Commons for functional photography & Flickr for random pictures of your cat)
- stories (including fan fiction? note fair use issues here)
- videos - could include documentaries for now since there is no better space; should exclude random YouTube style home videos
- game projects (i.e. new game ideas)
- 3D artwork/models
- borderline functional/artistic: fonts, clip art ..
- criterion of freedom - all works must be free works (including source data etc.)
- criterion of effort - is it a work worth preserving? (ratings will help here)
- criterion of function - does the work have artistic merit, or is it a work of knowledge?
On "adult content"
Follow the law where the server is hosted and basic ethics (incitement to hate, exploitation of minors, ..). Respect privacy rights - require appropriate permission. Resist censorship attempts. Judge erotica on its artistic merits, not on moral grounds.
2-3 decent servers should be enough for the first few months. May be a good idea to disable video, but Theora might limit bogus uploads.
- better discussion support - complete LiquidThreads
- file rating support
- better metadata (no starting requirement)
- media players
- importers (see below)
- automatic conversion of all uploaded files to free formats (e.g. Theora)
- generate & package dumps for BitTorrent distribution
For each of these, one could program specialized bots that authenticate as user X with password Y (to prove copyright & build community), then retrieve all the files from the user's profile:
- http://freesound.iua.upf.edu/ - may be switching to a reasonable license soon
- http://www.comicgenesis.com/ - how many artists could be convinced to use a free license?
- http://openclipart.org/ - yay for PD