It's been mentioned before, for example on Digg, but iTunes 7 has a new feature that counts the number of times you've skipped a track, which is defined as playing the song for at least two seconds but no longer than twenty (after which it is counted as played fully).
I'm curious how they chose twenty seconds as the cutoff - when I was working on an (unreleased and unfinished) project for generating smart shuffles, I defined skips as not listening past the first fifteen seconds, but didn't count a full play until the song ended.
It's an interesting feature, and it's apparently kept up to date by some iPods, so I can see it helping me keep bad songs off my small iPod by using it in a smart playlist rule. However, often I find myself skipping songs I really like because I'm just not feeling them at the moment. So - how about the option of a smart playlist rule that lets me define a skip percentage? If I skip a song far more often than I play it, then it's a pretty good bet I don't need it on my iPod.