Well several factors (some more to do with the company rather than the actual product itself).
- Worldwide sales/service.
- The happy price point (on the PP anyway, which includes a protective cover).
- Replaceable battery.
- Battery life (not to speak low of other readers but I see some people getting a few days to a week or two on a charge, my PP lasts longer).
- Number of options for page turn buttons, I can hold the PP in the most comfortable position and read.
- Standard cables/charger.