You should somehow incorporate user voting in your rating system.
Hello, we already have built in user reviews and ratings in our Bitcoin Exchange and Bitcoin Wallet reviews service:
http://bitcoinx.ioIt's a community driven service, where we do all the background research and data mining, and then provide it to the Bitcoin community. We've been around for about a year now, and have a large database of exchanges and wallets, with top rated exchanges and top rated wallets too.