You could already use Node through M4L. I'm not clear on what this adds that wasn't already possible.
M4L is basically a plugin sdk. It loads as a VST would (roughly), just with access to Ableton UI elements.
Ableton Extentions if a first class api to Live, kind of like AppleScript.
They made extensions first class, chose JavaScript as the primary language, and chose node.js as its runtime.
With M4L you need to implement your feature in a device and add it to your project. My Ableton project template has a bunch of these on my main track. With extensions you use a context menu as the entry point which will hopefully be more lightweight. Hopefully they'll expose more of the object model over time and let us trigger these via keyboard/midi shortcuts.