For me, perhaps, there would be two key features:
* Ability to access library works: with this you would cover all kind of DRM you find for ePub, so you can manage all your DRM-ed books.
* Ability to organize my books in tags/folders... as you like, so you can manage a lot of books.
My desire (not very important but something I would like a lot): to be able to see ALL the metadata as an option for each book
Terisa de Morgan