Michael Mandelbaum, a senior fellow at the Council of Foreign Relations, explains why Iraq is a more pressing target than North Korea in this Newsday article. He spells out all of the reasons, many of which have been mentioned elsewhere. The point I found most interesting, however, was this:
More importantly, a nuclear-armed North Korea, although hardly desirable, would pose a less grave threat to American interests than would a Hussein in possession of the bomb. For even a nuclear-armed North Korea could not intimidate, let alone conquer, its neighbors. China, Japan and South Korea are all prosperous, powerful countries with strong governments and formidable armed forces.
Japan and South Korea, although not nuclear-weapon states themselves, have solid alliances of long standing with the nuclear-armed United States.
In contrast, Hussein's neighbors, within whose borders is located much of the oil on which the global economy depends, are neither politically legitimate nor militarily powerful. They are no better able to defend themselves now than they were in 1991, when, after the Iraqi invasion of Kuwait, the United States led an international coalition that came to their rescue.
A nuclear-armed Hussein could intimidate or even occupy his neighbors, dominate the region, and hold the world hostage by the influence he would thereby exercise over its supply of oil. If he did possess nuclear weapons, the world would likely not be as quick or forceful in opposing him as it was in the early 1990s.
I don't recall seeing anyone who has compared Iraq to North Korea who has raised that issue. It is an important distinction, one that the anti-regime-change crowd ought to consider.