Fact is, most of the people that disagree with Iraq only express themselves in ways that are ignorant, stupid, or uninformed. I've seen very, very few people that can state in any remotely coherent form the administration policy that motivated the invasion of Iraq, let alone state coherent reasons for disagreeing with such. It takes too much effort; it's far easier to quote the latest soundbyte off the evening news.
Conversely, I don't know a hell of a lot of people who support the invasion of Iraq who can explain the administration policy coherently, either. In fact, I don't even think the administration explains their policies coherently. Most of the argument about Iraq as debated in the public domain basically consists of debating a policy that very few people not only fail to understand but don't even know exists because they haven't bothered to dig any more deeply than CNN or FoxSnooze - or, if they're reasonably curious, the Journal and NY Times.
But basically, I fail to see how anyone holding any opinion on Iraq in the absence of even a rudimentary understanding of the administration's global foreign policy agenda is anything but stupid, ignorant, or uninformed.