You could already use Node through M4L. I'm not clear on what this adds that wasn't already possible.

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.

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.