Honestly, I don't see why it is so amazing.
It functions like a fancy interactive shoutcast station, only the station/playlist is programmed by people who most likely have little to no experience on playing music for someone other than themselves. I don't see this being a success in the end. For one, it requires a ton of interactivity to function ideally. Something that people should not be doing while they work, which is when most people will be listening/ being a dj. It can't give you the well-rounded music listening experience that a shoutcast station can offer, and it can't give you what you want to hear like Pandora/Slacker can.
The only thing the site has going for it in my opinion is the interactivity, which a lot of people seem to love. But it is impractical for everyday listening. Perhaps I am alone in my thinking, but I don't want to play a game to hear music.
Not to mention, the DJ/Playlist aspect of playing music on-demand that you don't own costs a lot more in licensing. Substantially more, which makes me believe that a subscription or a microtransaction-like system will be required to survive. There has been a lot of talk about Pandora not being able to make ends meet, but this site will have to pay even more as they will have thousands of dj's to cover as well.