Rating posters encourages both good behavior and bad behavior. Check out Slashdot, which has a huge population of trolls and flamers whose goal is to get their ratings (karma points) as low as possible. Notice also that many of the highly rated posters spew the same, old party line since that is the reliable way increasing your ratings.

I do like the idea of rating threads for those people who can't spend much time on this site. However, most good threads also contain a lot of posts (ignoring flame threads, which also have a lot of posts), making them easy to find.

 

The new software has a thread rating feature.