The quality difference in small arms isn't that much of a gamebreaker. NK has kalashnikovs that are decent, reliable guns even without holographic sights and other accessories. The reason they are boned is because of the vehicles and big guns. Their tanks are outdated, their aircraft are no match for the most likely opponents and I doubt that they can stop an invasion coming from the sea either.
Well, the firearms difference is more pronounced than that. The typical North Korean soldier has a plain rifle with iron sights. Along with Americans and South Koreans having reflex sights for improved point shooting at close range and ACOGs and similar magnifying sights for improved long range shooting, at least four members of each 9-man South Korean squad have grenade launchers of some kind attached to their rifles that can be used immediately. Outside of hand grenades, the typical North Korean unit will usually present explosives in specialized roles or as dedicated teams; the RPG-7 was designed for a two-man team to use, in fact. Combined with the squad level usage of light machine guns and designated marksmen with scoped rifles, and the average Western squad simply has greater flexibility than units armed with mostly plain assault rifles with specialty weapons being applied as specific force multipliers in a grander battle scheme.
One of the biggest issues with North Korea when it comes to small arms is the training: they're focused on old Soviet doctrine that involves massive amounts of conscripts or even militia with bare levels of training to be simply thrown at the enemy en masse. They follow scripted battle plans with essentially zero flexibility, as they're reliant on high ranking decision makers. Individuality is discouraged in all segments of society, but it's especially bad in the military. Compare this to the US and similar militaries, which encourage individual initiative and a strong mid-level of non-commissioned officers who lead from the field. Part of this means that US soldiers are simply more effective man to man, as they're trained for individual marksmanship and skill rather than being given a rifle and pointed at the enemy.
This is also reflected in their choice of weapons: the average North Korean soldier has an AK that was designed for massed automatic fire and ease of use by poorly trained peasants, while the average American has an M16 or M4 that's been customized with some kind of optical sight and various other accessories (including grenade launchers and infrared lasers) that allow for them to achieve maximum efficiency and work to the best of their capabilities. We may have less than half the number of soldiers as them, but each of our men is worth far more than two of theirs.
IF their infantry is as fanatic as they would like us to believe they can cause the biggest resistance in urban areas where most of the fighting goes to infantry, so I agree with Chitoryu12 on that.
Not only residential areas, but the regular terrain as well. Desert Storm and Iraqi Freedom both involved direct routes to the targets across miles and miles of open desert, with the Coalition forces essentially driving in a straight line to Baghdad. The air force managed to destroy one armored vehicle with every four bombs or missiles used in Desert Storm, but this is because said vehicles were in large, open areas in clear weather. North Korea is very mountainous and the armor can be easily spread across difficult terrain where it can't be easily spotted and there may only be one direct avenue of approach to target it. The same terrain that prevents the North from easily invading the South works both ways.
Also, North Korea has large amounts of underground facilities, possibly even to the level of bases and command centers. Even if our bunker busters have the ability to destroy these facilities from above ground, you still need ground forces who can fight their way across the nation and find the hidden entrances wherever they are. These entrances, in turn, will be guarded by more ground forces. It'll be very much a land war.
Actually now that I think about this... The war would end when the remains of the NK troops surrender and come out of their holes and tunnels, but I'm not sure how long that would take.
That's the most likely scenario. A lot of the soldiers seem to only be in it for the "army first" attitude toward supply allocation and the added privileges of joining, rather than a mass fanaticism. There's a limited amount of capable officers and leaders, as they're usually "bred from the same stock": Kim Jong-un inherited the position from his father, who inherited leadership from his father. Government officials are selected based on loyalty and family connections rather than talent. Generals come from the commissioned officer corps, which in turn is rather small and mostly comes from dedicated officer training schools (and like government officials, success is based on loyalty to the party). There's no talented pool of NCOs that can lead squads in the absence of generals, or take over the commissioned officer positions as their leaders die. The less connection the soldiers have to their leadership, the less reason they have to continue fighting.
Maybe the whole deck of cards collapses few days into the war or maybe the fighting goes on for a year or few. The logical part in me says that their people should be glad to be free of that regime and that only ones who would defend it are either brainwashed or the few opressors who have lived in luxury during this regime.
Then again that is what some people thought about Iraq.
The Iraqi problems seem to have mostly stemmed from the problems caused by American occupation over the years rather than a genuine love of Saddam and his regime. North Korea is very much unlike Iraq in that it's a giant death camp: the majority of the population lives in squalor and starvation, with even cars being a rare luxury for anyone not in the government. Concentration camps are common and have large amounts of political prisoners and their innocent family occupying them. The borders are locked down and soldiers have even killed their comrades to try and escape to South Korea. Refugees almost regularly flee, risking immediate death if caught. The occupiers would probably provide the first regular meals they've gotten their entire lives.
I don't think they can keep the resistance going on as Iraq has. They get recruits from other countries and are fighting for a religion (or at least one version of it) but would foreign communists support north korean resistance? Do we have anyone outside that country that would say "No, I think their people had it better under the tyrannical pseudo-communist regime and I want to help them return to the glorious days of Kim Jong-Il."
Not anyone who matters. Every country will have its supporters, either the most fanatical (who probably have something a little wrong in the head) or ignorant foreign nuts who took the "Glorious Democratic People's Republic" propaganda seriously and want to see the rest of the world follow their example. But not even Iran supports them, and Iran is notoriously nutty.
Any thoughts on whether or not China might help us out? I think it's more likely than you might think.
China was the only ally North Korea has ever had, but that's over now. China supports the new sanctions on them, and were royally pissed by the nuclear tests earlier this year. The actual threats of starting nuclear warfare have apparently led them to wash their hands of the regime. At a minimum, China won't give them any help when the fighting starts. They'll be lucky if China doesn't outright donate troops and supplies to NATO instead.
China also has a desire to try and limit American presence nearby; we're both reliant on amicable relations and trading, but they really don't want us actually within arm's length. They also know that a unified Korea would mean that US forces are basically on their border. They may try to get permission to take North Korea as their own territory (similar to Taiwan) before South Korea can incorporate it.
She says China would do everything it can to prevent another Korean War, since in the worst case, North Korean refugees would go straight to China.
I don't think China has any qualms about closing their borders to refugees and even shooting them if they try to sneak through, but they can do nothing to stop a war. North Korea has gone off the deep end, and China knows that they're more trouble than it's worth. When you threaten nuclear war, people can't exactly defuse that conflict by trying to act as a negotiator. The DPRK's threats have escalated everything and essentially started the countdown to conflict.