I use the theme editor to quickly fix things, when I don't want to fire up my Code editor and FTP client.
Many times I find my self hitting Cmd+S (on a Mac) to save my changes, only to realise that I'm in a browser, which in these cases just offers me to save a copy of the complete page to my computer.
Wouldn't it be awesome if some JavaScript could catch the event of me hitting Cmd+S and would execute "Update File" automatically? I wouldn't mind if it's not AJAX, page reload is not an issue, it's the habit of Cmd+S that I really want to keep! :)
There could be a tick box in Settings, or right under the editor area, to say "[ ] Update file on Cmd+S / Ctrl + S" – and if it's ticked, only then would the JScript watch my thumb.