Compatibliity with epub and other generic formats.
NOT tied to any one book vendor etc.
6" or 7" eInk display with really good contrast and reasonably good grey scale.
Some sort of screen protection e.g. a cover.
Uses something reasonably standard (preferably SD card) to hold books.
Good battery life and preferably a user-replacable battery that doesn't require removing about 15 screws to get at it if it eventually dies.
STANDARD USB cable for file transfer / charing.
Not hugely expensive - I get nervous if I'm carrying really expensive kit.
Probably don't need Wifi - if I'm travelling I'll usually have a laptop with me.
Preferably available at a reasonable price from Amazon, since I got a lot of amazon credits for my birthday.