[frio]

  > you can pre-render the initial state of your app

This, I think, is the killer feature of Node, and the reason I'm slowly transitioning from Python for new web projects. You can reuse your server-side templates client-side (without worrying about, say, reimplementing Handlebars Helpers in your server-side language), and can easily render full HTML templates for the client that get enhanced when the client-side JS loads. This also solves UI nuisances -- like your server's markdown renderer being different to your client-side preview (grr).

Meteor and Derby are obviously heading down this path, and while I'm not sold on the rest of Node and the general JS style, having the same language in the browser and the server is too much to pass up.

[potench]

If you're coming from python, I'd recommend the 'swig' template library. Django syntax in node and client-side.