There's so many people here. I'd like to sort threads in a search by rating, but I can't because no one actually knows what's good? is that it? Because no one is rating threads, and if you do. It's your thread.
I think some people don't vote on threads because they forget that the feature is there. I've voted on my own threads before simply because (I think) that it will prompt others to vote on them too if they disagree with the current vote rating (one way or another). However, if there is no vote rating at all, then I think some people don't think they have anything to disagree with...or, as already mentioned, they simply forget that feature exists.
For months the only way I've been able to vote on threads is by posting with the advanced form, and selecting the rating there. Which is lame considering many threads don't deserve an extra post just to down-vote when the author thought a bit too highly of their thread. :tumblewee