RE: Differences in response to NK aggression and Iran aggression.
The main difference is we actually fought a long, bloody war in Korea, and ultimately ended up in a stalemate position. Primarily, because China views NK as a buffer state to prevent American troops on a land border with China. This resulted in China pouring an immense amount of resources into the country during the war, and propping it up afterwards. It is entirely a buffer state. Iran has no such neighbor to give them an incredible amount of aid. Sure, China and Russia help them to an extent, but Iran doesn't serve the same geopolitical function for a China or Russia that NK does. That is why NK is allowed to exist in a weird limbo state.
Also of course, because SK could be invaded by NK and almost completely obliterated in a doomsday scenario, so the costs for a war are significantly higher. American (or US ally) interests don't directly border Iran, so there isn't a concern that the Iranian military would invade anything.