My primary purposes for an eReader is for "novels", so I want a device that maximizes that experience.
Large page forward buttons on both left and right sides, toward the lower half.
Good customizable reading software. Should be able to choose font with any point size, margins, line spacing, spacing between paragraphs, ...
I think an 8" device would be my sweet spot for size, and nothing smaller than six inches.
Folders, card slot.
The higher the resolution of the display the better.