I'm the author of the CoffeeScript book from PragProg, which includes a chapter on Node.js.
I've done a fair amount of server-side development in Java and Ruby. What I like most about Node.js is the feeling that you can be as close to the metal as you like. You don't have the endless boilerplate of every Java web framework, or the extensive "magic" of RoR. With Node and a framework like Express, you can understand things at the level of "let me look at this HTTP request and decide how to respond to it." This, to me, is beautiful:
It's going to take more time to build a full-featured web app that way than with RoR or Django, but you'll understand it better and it'll scale well. For something that's only been around for two years, Node.js has a remarkable ecosystem—you can tell that a lot of very smart people are gravitating to it.