Waldman points them out:

There are some things we can confidently predict about the town hall debate. Obama will almost certainly arrive more awake and aggressive than he was in the first debate. When Romney gets a question he has anticipated, he will deliver a confident, well-rehearsed response. But it's the unpredictable moment—the oddly phrased question, the out-of-left-field topic, the voter's personal story—that will likely define the debate. And that could be Romney's real test.