Here are a few things that crossed my mind right off when I started using it:
--It would be nice to have the ability to store "bookshelves" in folders in the Collections area.
--The ability to export your library in a navigable format (so you can show friends what you have in your library and they can pick what they want to borrow) with options for: .pdf; .rtf; .txt; .html; and perhaps even provide integration with iWeb, it would be a cool extra feature that would get some people's attention I think. =)
How do those sound?