I have been considering making one. Goodreads is so awful, and the alternatives I have found are all quite quirky or broken in one way or another.

But having a database of "all books" is not necessarily trivial. Even though OpenLibrary really does provide a great start, the contents seem to come from Goodreads/Amazon in a lot of cases, and I'm concerned about the legality of making a commercial competitor based on it.

Also, it would take a lot of time and data to get a good recommendation engine going. Amazon really is in the best position to do this. Just a shame that Goodreads get so little love from them.