Prime importance - absolute top - is USABILITY.
This device has to be comfortable to hold, and sensible and intuative to use. No reaching or stretching or changing hand position to press button, no endless menu options, I want something that I can pick up and read for hours without fuss. It has to be robust, and longlasting.
Other useful things, Helpful, available tech support, full manuals, reasonable software that sorts sensibly etc
Only after that do I care about looks, brand, colour and fancy features or whatever .
I don't want touchscreen. Particularly not if it comes at a cost of screen readability. Ditto wireless or 3G. (but if you can make internet proxy so it can buy without geo restrictions I'll reconsider
One last thing. I want a reader to read, so that's all I want it to do. I don't need it to be a phone, a camera, or a gps. I don't want to get on the internet, check my email, do my banking, or watch video. I want a dedicated device that lets me read comfortably and easily.