Korea's also different to Vietnam/Iraq in that NK's biggest buddy, China, probably won't even be willing to provide the arms and suicidal zealots needed to efficiently carry out the constant low-level attacks that constitute Gorilla warfare. NK doesn't have a religion enticing foreigners to come and kill the invaders (ala Iraq) and China is only moderately likely to want to keep NK around as a communist state buddy, so they aren't likely going to want to send arms down like they might've perhaps done during the cold war, and especially not soldiers like the first Korean war. It's probable that the Guerrilla warfare would run out of committed participants after a year or two, or hell, hopefully even never eventuate, like the guerrilla warfare Hitler promised after the fall of the Reich (admittedly, that's a total pipe dream. The Nazis didn't have 60 years of social isolation to indoctrinate the population and the years of war had allready sapped the will to fight out of the starving and terrorized population).
The fact that most of the occupation and footwork would be done by south Koreans, who speak the same language is an extra bonus. Soldiers who can be understood and spoken to are much easier to identify with and thus, somewhat more morally troubling to kill. It's a little harder to not see them as aliens to be killed if you can hear and understand what they're saying.
(4 posts while I was typing. Aqizzar's got a good point)