I had this quick idea, so thought i'd record it somewhere..
As well as the regular 'timed' autosaves, we could have something which detects that no input has happened in the editor for 3/4 seconds and triggers an auto-save.
The purposes of this would be to trigger at natural pauses/finishing points that people have when typing without having constant saves going on in the server. It would also mean a save is done as soon as they are 'done' with the editor (maybe spend a few mins messing around with other form settings).
I think it has a potential fringe benefit of also making the use able to see the autosave has happened (which I think is a key part of the feature - to give them confidence in their text being saved).