In my view Obama got by far the best of this exchange.

Obama:

Iran, Cuba, Venezuela –- these countries are tiny compared to the Soviet Union. They don't pose a serious threat to us the way the Soviet Union posed a threat to us. And yet we were willing to talk to the Soviet Union at the time when they were saying we're going to wipe you off the planet.

McCain:

Obviously, Iran isn't a superpower and doesn't possess the military power the Soviet Union had. But that does not mean that the threat posed by Iran is insignificant. On the contrary, right now Iran provides some of the deadliest explosive devices used in Iraq to kill our soldiers. They are the chief sponsor of Shia extremists in Iraq, and terrorist organizations in the Middle East. And their president, who has called Israel a 'stinking corpse,' has repeatedly made clear his government's commitment to Israel’s destruction. Most worrying, Iran is intent on acquiring nuclear weapons.

It seems pretty obvious that Obama gets it, and McCain doesn't. In fact, McCain's entire campaign rests on the idea that he can inflate Iran into such a big monster that voters will flock to him to defend them against it. Meanwhile, if Iran is such a menace, according to McCain, doesn't that make it even more important to talk to Iran?