Using the same language on the server and the client
Event-driven asynchronous programming is super fast
The apis that node implements makes creating asynchronous applications very easy. Even though it technically only has one main event loop, they're process api makes it very easy to use your node application on all cores on your box in a very cross platform way that does not involved the main event loop to process requests. They all listen on the same port and allow the kernel/OS basically round robin the requests. Nice.
The community is strong and they have some really cool projects
A few that are interesting to me is the Express framework to implement a MVC architecture. Also some of the templating systems, like jade, are quite refreshing to use (at least compared to my experience with python template systems). Also the Socket.IO library seems to be quite complete as well. Lastly all of the session management, memcache, db, nosql libraries are availablie with the npm package manager.