Hi,
I had a similar thought and decided to give each song I like a rating from 1-10 in the BPM field (1 slowest, 10 fastest) regardless of genre, artist etc. Then just create a couple of Smart Playlists which select ranges of BPM (or whichever field you choose) depending on the mood you're in eg the "Gym" smart playlist would have BPM between 8-10
You can also add letters into the Comments field to further select the music, so to exclude Christmas songs for example where every Christmas song has an X in the comments box, tell the playlist to ignore where Comments = "X".
It takes a long plane/train journey to assign all the values but once you've done it, and keep up doing it when adding new albums, you never really need to choose what to listen to again if you're stuck
Macbook Pro C2D, 2.33GHz Mac OS X (10.4.9)
Macbook Pro C2D, 2.33GHz Mac OS X (10.4.8)