(a.k.a. shameless plug)
Since the introduction of the 'smart playlist' I have spent far more time than is healthy playing with iTunes - creating sub playlists depending on genre, rating, playcount, when last played, when purchased, length, inclusion of certain words in the title etc, and then blending these together to make the ultimate playlist for my day. Geeky, hell yes. But I enjoy it and enjoy the fruits of my labour whilst commuting.
One thing that is hard though, is sorting by mood, or 'upbeat/low key' songs. It would appear this problem is solveable in a way other than trawling though every song putting 'high' or 'low' in the comments box.
TANGERINE, is a funky little app (still in beta) that took just under an hour to sort through my 3000 songs and rated them by BPM and intensity. How cool! Essentially, you can pick a range of BPM or BI values and use that as a criteria in your playlist creation. So now, I can have 'keeping me awake early in morning when i have to work' as well as 'chilled out because i'm in danger of going mad from stress' playlists!
My highest intensity song appears to be Can You Feel It (Club Mix) by Matrix and lowest is Cirith Ungol from The Return of The King soundtrack... eclectic...
Think it is only available for mac at the moment, but if you are using a mac with itunes have a look at it while it's still in beta. Apparently, if you blog about it, you get a free licence when the full version comes out, hence the out of character plugging...