To answer Marcus's question: there are about 300 out of 570 total reviewers wrote only one review. If you ask me why, I can only guess and I think your reasons all could be valid. People are lazy.

I was acctually considering the idea proposed by boccaccio. The question is how to setup the system. And again, how to do the quality control. You can't just say 'good story' to become a top reviewer.