I stumbled across a great Spotify feature the other day. If you right click on a playlist, there is an option called "Create similar playlist". I did this with a playlist I had created from a bunch of random tracks. It basically went through track by track and found similar songs. It was a bit hit or miss on some songs, but a neat feature. I think Spotify does a great job because they give you so many options for finding new music, but sometimes you have to dig into the UI to find them.