To me, he looked angry. Furious.
You are onto something. Obama has a temper. Remember this interview:
President Obama chides reporter from Texas TV station
I am convinced this is Obama's Achilles heel. In a debate, it could be his undoing. Find the issue which triggers the anger, hit it again, and again and again. Obama looses his "cool". Then after the debate, drive home the point about Obama loosing his cool, and his image as the cool, unemotional, "Mr Spock" starts to unravel.