It seems like it would be a very useful thing to be able to fire off a sequence of chained reactions in response to an event.
For instance, this is specific to home automation, but the concept could apply to many other things:
Event: Speech Recognition "Goodbye house"
Reaction 1: Respond with Text to Speech "Goodbye Michael"
Reaction 2: Turn off all lights except entry light
Reaction 3: Reset thermostat
Reaction 4: Delay 5 minutes > Turn of Entry light
I'd write my own add-in for this, except Mayhem's UI wouldn't support it.