I've daydreamed up some similar website concepts, but just haven't had the motivation to make it happen. There's already Goodreads.com, but I find it a little disorganized for my taste, and I've also had Shelfari.com suggested to me, but haven't checked it out yet.
The part that seems really important to me, which none of these sites seem to get, is browsing. It should be easy and convenient for a reader to burrow down into a specific genre or style of book, and start leafing through titles and short summaries. Instead, they give me ungainly lists based on broad distinctions, or user generated my favorite whatever lists, and I can't be bothered to sort through either.
Ultimately, the right sort of site will pop up, but I think it'll have to be the authors themselves who start it because they're the ones with something at stake. The ebook market is turning them all into entrepreneurs, and their business depends entirely on being able to find and connect with an audience. Once they realize that no one else will do it for them, and that nobody else has their well-being in mind, they'll find a way to band together and make it happen. I think.
Well... I can dream, right?