You start a station based on what genres you like, then you start morphing your station by rating each selection by artist, album, and song.
Over the past few months I've listened as my electronic station morphed a into dance > ambient > dub > downtempo playlist. It's great...I waste a lot of time just searching their database and rating stuff.
One 80s based stations started playing Brian Collins, Michael Jackson, etc. Bad Echo! Bad! I put a stop to that little tangent.
Shoutcast has some gems...live365 sometimes, but a worse S/N ratio imo, lotsa variety though. now I'm gonna checkout monkey radio when the mood strikes.