The same was true for YouTube comments, and that's why they switched to Like/Dislike buttons. When it comes to judging the value of things, people tend to be all or nothing.

That's more the case if you show some sort of aggregate 5 star rating on the front page, I suspect. If you look at peoples iTunes libraries they probably use 2-4 stars a lot and 1 or 5 stars more rarely. But on Amazon, people who think a book is 4 stars but see it rated as 3 stars are going to vote for 5 stars to push it up more.

They should aggregate the reviews by taking a median. That way there's no incentive to game the system.

Only works if people understand there's no incentive...

Which they won't, cause math is so hard...

Only the median voter(s) would need to understand that there's no incentive.

> When it comes to judging the value of things, people tend to be all or nothing.

Not really. The sample is self-selecting. People who neither love nor hate don't care enough to vote.