Version controlling for design is great but if you want to differentiate from layervault or pixelnovel.com then you should try and see if designers would collaborate in an opensource fashion, ala github. (I am really curious about this, as that's the real reason people use github AFAICT)
So I would go and add some social aspect for it, make it a bit more like dribbble, behance, flickr (portfolios, inspiration) as these are the sites that designers use.
Also adding stockphoto/illustration selling and job board features would probably make for the best designer platform ever. (not mentioning the business options...)
And after this feature suggesting madness, I would like to point out that if you want to keep this simple and stupid, then still please get a designer and work on the ux.
Keep up the good work!