See... we did a preemptive strike in Iraq b/c we are seeking to stop terrible weapons from being developed there (i.e. nukes). So, it is fairly easy to go in and own them before they have big, nasty atomic bombs which they could use in retaliation. A "relatively" straightforward military confrontation.
NK on the other hand... likely already has a nuke or two, and wouldn't think twice about setting one down about 40 miles south of the border in Seoul. I've been there... I was doing work at a company real close to the DMZ.... tanks and military along the highway.... it was weird. Anyways.... that would be a complete and utter tradgedy if a nuke were to land in Seoul. Depending on yield of the bomb, millions could/would die. So, I think NK is a totally different beast and needs to be handled in a totally different way.
Oh... and NK has an army about 4x as large (if not more) as Iraq, plus slightly better weapons. Not to mention it is widely thought they have a long range ballistic missle capable of delivering a warhead to Bill Gates home in Washington!!!