I've got my preferences, but I don't think there's really anything that would make me outright reject a reader. In general, I'm not a fan of:
-Touch screens. Not so much because I fear fingerprints but because I dislike the texture of the e-ink screen. It's a tactile thing for me.
-Too many steps for writing annotations and notes. My first choice is to have a physical keyboard ready and waiting, but if a touch screen is the only option, I'd want one that brings up a virtual keyboard in a single step.
-Requires a computer to download books. I want WiFi at a minimum, but 3G is even better. Bonus points if it has a browser; I like being able to look things up on wikipedia.
-LCD screen on a dedicated reader. I don't mind reading ebooks between rounds of Angry Birds, but for sustained reading, e-ink is where it's at!
-Crappy battery-life (see LCD screen)
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