Edit: got blocked by AdBlock+.
Remark: it ignores characters/keys that aren't present on US keyboards. No é è à ñ ö π ς φ ... would it be possible to catch Alt/Shift keys, for instance?
As far as other modifiers, I think a lot of those aren't available to the keypress handler (Specifically, things like cmd-* on a Mac are usually handled by the windowmanager or as app menu shortcuts). I think I have seen html5/js demos that capture more than the usual set of keystrokes though (emacs & vim js demos spring to mind), so it might be possible to do so.
As far as special characters, that'd require you to detect the user keyboard layout in order to know where the appropriate key lives (with added effort for dead-key style modification)
Edit: poking around in the http://www.ymacs.org/ source, there's a reference to http://unixpapa.com/js/key.html which gives access to a whole bunch of extra keycodes.