Here are my thoughts.
Iraq is totally different from North Korea. In fact the whole idea of getting rid of Saddam is to prevent him from becoming a North Korea. Imagine what Saddam would do if he had nukes. He could say Israel must withdraw from all the West Bank or they will face a Nuclear strike. He would get plenty of support from the Arab world. But can you picture what the Israeli response would be? Total war. He could threaten Saudi oil fields again just like he did before the Gulf War, threatening the world's supply of oil. Instead of just tanks this time he has nukes. He could then pressure every country around him and if they don't go along they will face annihilation
As it stands Saddam is isolated. He can't truly bring his fight on the regional stage because most of those around him don't like him. It's better to remove him now than wait for the job to get even tougher.
North Korea on the other hand is able to bring the fight to a regional level. They are using their weapons to blackmail the world community. This game is not only regional but global. As mentioned earlier, North Korea and Japan have no love between them. If a war does break out and North Korea nukes or guns after Japan it will not only kill millions of people but cripple Economics for the region. That will filter into the world. Think about it. How many parts on your computer, TV, VCR, DVD, CD player, Stereo, and any other electronical device, was either made in Japan or the technology was developed there? When the stakes are this high you must play the game with much more finesse.