I think a lot of the growth is/will be in offices where people are working at computers. A group of people with headphones at workstations DJing to each other is very compelling, and turns a part of the environment that can be isolating into something social. Likewise, friends working at different companies can be plugged in to each other in an ambient way while working. That's the coolest part so far.
Beyond that, there seems to be a lot of ad-less Pandora going on: join a genre-focused room and get a stream of stuff you may or may not know without the labor of building a playlist in Grooveshark. As stated elsewhere, this can be annoying though, because people suck at sticking to the stated genre (ie. frantic Drum and Bass in a "chillout" room).
If it really gets traction, it will also become a target for bands to market themselves. I've already seen some DJs spinning their own songs.
Big hurdle: it's ugly. They should fix that, especially since it's basically a "place" to hang out.