I would argue it's an Amazon referral engine framing itself as a book review site. They have optimized for purchases and not for reader happiness / quality.
A competing site that is focused on surfacing books that the reader likes, with links to more than just a single purchase point would likely gain enough traction to be useful.
I imagine that scraping a user's GoodReads profile every now and again with authorization would also allow the user to update status in their Kindle / eReader while still populating another site, which would be interesting.
You could argue that is necessary to get engagement from reviewers, but in trying to be both, goodreads doesn't do either very well. Throw in the fact that they haven't improved much since Amazon acquired them, and they've become a sort of static site targeted at acquisitions for ebooks.