We're holding back on striking North Korea not primarily because of what NK can do to us, but because of what NK and do to South Korea. The Seoul, South Korea metro area is located only 20 miles south of the DMZ. North Korea has thousands of fortified heavy artillery pieces, which can all reach Seoul, dug into the hills just north of the DMZ. NK has the capability to rain down tens of thousands of H&E artillery shells per hour on Seoul and its 10 million residents. This is the main reason we haven't struck NK's nuclear facilities yet. Kim Jong Il is basically an extortionist who holds South Korea hostage with his heavy artillery to prevent the US from removing his regime.
If I remember my history correctly, the inital Allied counter-attack in the Korean War pushed NK forces well over a hundred miles north of Seoul....out of artillery range. But Mao Tse Tung brought China into the war and pushed the allies back to the present DMZ. So China screwed up the Korean Peninsula and is responsible for the current unstable situation there. Ironically, Mao's most talented son was killed in a bomb and napalm attack by Allied planes in the final weeks of the war. (Was that Ted Williams or Jerry Coleman in that plane?)
I have read that NK has mostly outdated artillery pieces, 25 MiGs and older issue AK's, T-34/54/55/62 tanks the newest of them dating back to the '60s. The soviets supposedly sold them an entire WWII-ear artillery production factory that is actually still in operation today! Not much upgrading since the '80s. A modernized SK military could probably defeat them without our assistance.