My biggest complaints are I have to log in each time I check out a book and remove from wishlist does not work.
My minor complaints are going to Available now by default, Not realizing the check out limit has been reached when on book details page. Having to change my check out length settings each time I need to check out a book.
One big annoyance was every time adding a book on hold it took me to bookshelf now it takes me to log in so I can't complain about that one till the other is fixed.
