Rarely the pitch involves being able to share libraries between the server side and browser side (wonderful benefit of node.js)
Most of the time it's being sold as "you already know javascript" or "it's super fast, because non-blocking is magic sauce!"
I haven't used Node.js for anything serious but the hype and especially the discussion around the now infamous node is cancer post sure makes it seem like Biilmann was correct when he said "For better or worse, Node is like the PHP of concurrency and ever so often worse is better"
Even if that's not true, if that demographic is the dominant users of and contributors to Node then that's what Node will become.