Specifically the fact that every eReader I've used (and I've used many) displays some sort of image when it is "sleeping." Amazon even sells Kindles that show ads when the device isn't being used.
Why not display the cover of the book currently being read on the device when it is sleeping? I love book covers, and I miss seeing them when I'm reading something, so why display them?
This would have to be an optional setting, though, because one of the great things about eReaders is that you can read super trashy novels in public and no one ever knows (not that I would do such a thing).