We all know they have nukes and might be crazy enough to use them. What is less obvious is that the country is a mess that none of its neighbors want to get stuck with cleaning up.
South Koreans have seen what a drag post-reunification East Germany was (and is) on West Germans. However East Germany was in much better shape than North Korea. Therefore South Koreans believe in reunification someday, but not anytime soon.
The other neighbors, mostly China but also Russia and perhaps even Japan, don't want an economic and humanitarian catastrophe in their backyards. A North Korea meltdown would send an unwanted wave of starving Koreans across the Chinese border, leaving the Chinese with two equally unpalatable alternatives, turning them back militarily - a public relations disaster - or taking them in as refugees - an economic disaster.
