I still prefer PHP / Ruby for a serious application, but, Node JS (Geddy / Express / Meteor / Restify) can make it really quick to set up a prototype UI or API with some Mongo storage.

* One Language : If you're going to be using a whole bunch of AJAX or Websocket style long polling, why not keep as much as you can in one language? Might be easier in the future for training.

* Event Driven : Because you start expecting events and callbacks, it's a different style of organization that you might confuse when working in more procedurally processed languages (PHP).