I just looked at this thread and, wow, what an amazing number of different viewpoints -- which is to be expected, I suppose. I've been lucky enough to have visited Seoul several times so let me tell you some things the people who live there are concerned about:
1) approx. 25% of all S. Koreans live in Seoul. That's a lot and it's because most of the country is quite mountainous.
2) all of Seoul is within simple artillery range of the northern side of the DMZ. No one in the south actually expects an invasion although there have been some truly amazing tunnels discovered (one was filled with T- tanks, 3 wide and that's some tunnel). The ballistic missles are an ego and PR exercise/demonstration much like the Red Square parades. S. Koreans are concerned primarily with a first strike by conventional artillery and short-range surface-to-surface missiles -- of which the North has a lot. A simple artillery shell doesn't have to be filled with explosives: when the US and former Soviet Union signed on to the Biological Weapons Convention, the majority of biological weapons that were destroyed from stockpiles were American -- ingenous design artillery shells, two-chambered with a shear-unstable separation film that allowed the sprial trajectory to mix two chemicals and then explode 200 feet in the air as a cloud of Sarin gas. N. Korea has acceded to the treaty but ...
3) it is, by definition, hard to predict the behavior of whackos. Even China is presently concerned with the military in N. Korea and that's significant. China's interest is purely self-serving; the part of China that borders N. Korea has more ethnic Koreans than Han Chinese living there and it's in Beijing's interest not to have an immigrant flood of foreigners.
4) Ever seen that old movie, "The Mouse That Roared"? That's the frightening scenario (the first part of the plot, not the comedic twist)