A good, fast internal search engine, that can also handle misspellings. I almost always do a search when I'm looking for a book online. It's crucial that it works well.
Results should be displayed in a manner that gives a good overview and crucial details such as avalable formats and price.
I should be able to customise my bookshelf, to display in a certain order - perhaps also have several views or bookshelves - then I could choose to share some of these bookshelves publicly and keep other private. Wishlists could be organised in the same manner.
Preview/excerpt download for each format is a great idea - and then ability to access the bookstore online and buy the book at the end of the sample so I can continue reading. This requires ability of online access from the device of course.
And obviously no DRM or geographic restrictions, and resonable prices.