The ability to assing a "modifier only double keypress" as a key binding.
"keys": ["control+control"] = Typing the control key twice-in-a-row executes the command.
This would be really handy for the Control Palette.
It's a nice way to get another "easy target" keyboard shortcut after all of the "space+___" shortcuts have been taken.
I first saw this feature in Quicksilver and it's very convenient.
Here's some related links and information:
Customer support service by UserEcho