I've got a few smart playlists set up that are restricted to tracks that haven't been played very often. You would think with random shuffle and 6000 songs that one wouldn't hear the same things time and time again, but you would be wrong. Random really is, and unless somehow you run through the whole library without resetting the random playlist there's a good chance there are tracks you'll never hear. Not that easy; if you plug in to iTunes, it resets the random playlist. If you don't listen for 3-4 days, the iPod powers down and the random playlist resets when it comes back up. If you want to listen to something specific, the random playlist resets when you go back to it. These restricted playlists are really cool if you want to listen to stuff you haven't heard in ages, or if you want to avoid repeats.
So that all I knew.
What I didn't know is that iPod smart playlists are only updated when you plug the iPod into iTunes. They don't update on the fly, like they do in iTunes. "I just got a repeat of a song that came up yesterday." "My 'never played' list is still showing 1200 songs, but I listened to 200 over the weekend." If you never plug it into a computer, they don't update. Plug in. "Wow, my 'never played' list just dropped to 1000 songs." You get the drift.