. Something simple, no fancy design. Big screen (in relation to the device size) with minimal frame and case. But easy to hold with one hand and press the buttons.
. High degree of configurability. Allow the user to set a custom CSS sheet (with margins, fonts, etc.), portrait or landscape orientation, optional status bar, arbitrary zoom factor, auto-scale images to fit the screen (optional too), dictionary lookup, annotations (even if it's only select a word and mark it with a sign). Full ePUB support with multilevel table of contens, smallcaps, headers/footers, SVG... Decent PDF support with antialiasing, hyperlinks (intra-document), two-column reading, etc. Possibility of turning the device off, but leaving something in the screen (a picture, a map, a checklist...)
. I'm very happy with my Cybook Gen3, so that would be pretty much my choice. Faster processor, more memory, support for SDHC, or even USB drives.
. Something dark (black or dark gray), something light (white or light gray), no need for more colous, but have some nice skins available
. Frequent software ugrades, possibility of additional apps, even user-contributed.