The biggest reason on "Why Don't I Want to Attack North Korea" -list is that regardless of the debatable quality of their air force, navy or even army, they've got a huge amount of artillery piled behind the border, in range of Söul, the capital of South Korea.
And if someone tried something against them, the first operation the NK government would order is to quite completely wipe out Söul. That would end up with thousands of civilian casualties, and I doubt even craziests war hawks in US Government would want that.
So... from a military POV, a war against NK could be "won" quite easily - as long as it's technology vs technology, the North Koreans don't stand a chance. Their logistics would be crushed quite fast. Then it would be just guerilla war, and I don't have any idea how long the NK soldiers would want to keep that up... but there would of course be some nutcrack commie minority who would keep fighting until they die in some cave when old age finally gets them.
The problem in this kind of war would, obviously, be "collateral" damage - even moreso than on latest Iraq incident or perhaps even the Israel vs Hizbollah struggle on Lebanon. 
No.
NK has lots of artillery, and that's certain. A large part of that artillery is obsolete WW2-era stuff, like famous Soviet 122mm pieces. Because of it - wear, tear and age all affect the power of artillery which is best used in concentrated efforts - the efficiency of NK artillery propably is pretty bad. This is regimental and divisional artilellery. NK also has rocket launchers like every other country with powerful military. These are quite handy tools, if they have long range and sufficient numbers. Sufficient numbers are easy to reach if you go for cheaper Katyusha models which are quite effective in large amounts. Range is far more difficult. NK goes for pretty standard Katyusha clones, which lack in range (RLs usually have longer range than artillery pieces). Bulk of NK ari is not capable of reaching Seoul. Unguided rockets usually have even worse CEP than unguided shells and as such are effective in large numbers. Increasing range also means lessening the payload, because rockets carry their fuel in the projectile.
So what? Well, the long-range rockets are one of the two backbones of long-range NK artillery that actually has the capability of attacking Seuol. And mind you - not the city center, propably just the northern outskirts. Granted, NK has these but nowhere in numbers required to demolish Seoul. So they might terrorize the area if they could. NK also has another weapon, and this is big-ass artillery guns - over 200mm in caliber and using LR munitions. These guns are pretty cool on paper, but they are hardly mobile at all. NK has most of these in fixed positions because caves provide cover and fixed position provides accuracy. However, they are fixed targets. Fixed targets do not last very long in modern warfare, especially if your enemy has air superiority or radars.
Artillery and adjacent systems lobs solid objects onto trajectory. If you can track the shell, you can easily find out where the shell was shot from (this is why the focus has switched from classic guns to SPA and stuff). ROK, US and most other technologically advanced militaries have the ability to detect this incoming fire and then launch counterbattery fire. This is actually pretty big thing in ROK artillery and they are quite good at it, or so I've heard. A fixed position can be targeted either by indirect fire, or missiles, or aircraft. Moving assets are trickier, but once they fire one salvo they have to switch position or risk their lives.
The bulk of Nk artillery is unable to reach even the outskirts of Seoul. The ones that are able are few in numbers and pretty easily destroyed (propably one of the highest priorities in case a war break out?). "Demolishing Seoul" is interesting myth and pretty good at keeping people uneasy about war, but it is more or less a myth.
