Hmmmm....I am not a gadget man.
So my requirements would not be too high, I think
8" eInk screen.
Battery life at least on par with PRS-505, or better.
Dictionary support.
Search function.
Bugless software.
Option to read self-created books (like I do now on prs-505, I create LRFs up to my standards).
Maybe...just maybe, not sure if it is really needed - a user-replaceable battery.
Cannot think of anything else right now, if I have forgotten something, I will add it later.
[EDIT]Addendum
Very important:
Page numbering is essencial (like in Words, PDF). No progress bars.