Everyone once in a while, when I'm listening to my iPod on shuffle a song comes on that I really hate or doesn't play right. The problem is I never remember to actually delete the song. If the iPod had a trashcan, I could just add the song to the trash, which could be automatically emptied when I sync. Or better yet, I could empty the trash on the iPod itself.
Trashing a song could be done through the rating interface, by adding a trashcan icon before the stars. That way rolling all the way counter clockwise would mark it for the trash. This even makes intuitive sense, as if the song really sucks (i.e. low rating) it should probably be deleted. Of course, that would make it hard to trash a song and keep it's rating...but hey that's for Apple to figure out.