Having owned a Sony 505 for a week now, let me start by saying I like the Sony, however...
I wouldn't mind a bigger screen (8 inches would be fine, but no huge border).
Better contrast
A built in light
Better software
When I go into a B&M book store, I don't find Stephen King under "S"
Why can't I pick border size for a page (and I don't want to hear "it's the file format")
More font size selections (finer granularity)
Why not more customization (why doesn't the bottom right direction pad page forward)
Maybe the idea of users (for things like bookmarks, continue reading)
Better mechanicals - I sometimes have to push a button twice