I guess you guys above have not read Harry Potter or Twilight! Those are my most popular, with over 1 million ratings apiece.
As for the ratings, I find the "outliers" very interesting: where I have rated a book very high and the average rating is very low, or vice versa. My outliers:
Divergent by Veronica Roth: 2 stars by me, 4.39 avg
Girls Guide to Hunting and Fishing: 4 stars by me, 3.12 avg. I loved that book and I don't even like chick lit very much. I see a lot of people were really annoyed by it.
None of the books my list have less than 3 stars average. Girls Guide to Hunting and Fishing was the lowest rated on my list (sad!). The next two lowest with 3.13--The Particular Sadness of Lemon Cake by Aimee Bender and The Vintage Caper by Peter Mayle--I also really hated.
My least-rated book is Echoes from the Macabre by Daphne DuMaurier which I believe is out of print.