Awesome poll idea ^^
Being able to read any book from your personal e-library at any time, anywhere. Also the convenience in general in relation with pbooks, i.e. size, weight and holding and keeping it open.
Also the ability to select our preferred font size is nice. I would consider the internet factor but sometimes I look at books at book sections of supermarkets or bookstores shelfs and when I get home I'm going to see online the ones that interested me and see their availability on Kindle.
Also you forgot one important one...