I wonder what the average account age and purchase total is of genuine amazon accounts. Seems like setting some sort of age and purchase threshold to reviews could make the fake review game a lot more difficult.
You could go all the way with this: look at my reviews, and then only show me reviews made by people with similar tastes.
If you check my Amazon account, I don't even rate most of my purchases.
It's a rather low sensitivity metric but one that has some value.