He's made this country too comfortable, and he doesn't stand up well to criticism without a war or terrorism to fight. I think that the N. Korea thing will be fixed within the next 6 months, and if Iraq, Afghanistan, and, most importantly, the economy, haven't been handled well at that point he's in trouble. That would leave an entire year of campaigning in which democrats can harp on all of these problems. There just seems like there will be too many problems at the time of the next election.
On the other hand, the economy is looking better (in the past few weeks) post invasion, and if there is anything that helps an incumbent, it's improving prospects for the future of the economy. Remember that most elections are ruled by one key issue, and if things are looking up at the time of the election, he will win. Plus, people will think he's a world beater if he pulls off this democratic transition without the UN screwing it up.