I'll give some insight into how I personally pick the games I rate, maybe others do it in the same way. First, I make it a point to always play and rate the people who give helpful comments with good feedback in them. Also, I sort by Most Karma to find the people that have been rating other's the most and play and rate their games. It is impossible for me to rate every game, so I give more attention to the people who have been spending the effort to rate a lot of other people's games.