You raise a good point about mood tracking. I've found that iTunes's genius shuffle does a decent job of shuffling a single genre of music, so you don't general

You raise a good point about mood tracking. I've found that iTunes's genius shuffle does a decent job of shuffling a single genre of music, so you don't generally get harsh interruptions. The holy grail is most definitely a library (or app) where you click "shuffle all" and enjoy every song it plays. Songza, genius shuffle, 8tracks etc. all try to tackle this, but I still think we're far from that point.