One problem that really interests me is the "dynamic playlist." Once your music library becomes sufficiently large and diverse, simply shuffling songs ceases to be an acceptable way to listen to music. You'll randomly switch between vastly different genres, come across tracks that aren't enjoyable outside the context of an album, jump in halfway through some pieces. It's a mess.

I wrote a simple script which uses Last.fm data to generate a "path" through the artists in my music library based on their similarities. It's very far from perfect, but it suffices to build an album playlist which slowly takes me through several genres. Some day I'll improve it to work based on albums rather than artists.

Ideally, I'd like to be able to run queries on my music library and have an interesting playlist returned to me.