There's another issue that also doesn't come up much: there's a certain fraction of the population which wasn't in favor of the Iraq war, and they like saying "I told you so" but in fact they didn't. Most of their argument was meaningless slogans. Iraq wasn't about blood for oil. And there wasn't an intrinsic moral problem with it.

I'd be careful about that kind of thinking. When you get something wrong, dismissing the people who got it right as low status and wrongheaded is a convenient way to avoid updating on the evidence. A bit under half the US population opposed the Iraq war, including many prominent people (Senators, opinion writers, policy analysts, etc.), but you seem to be focusing on a small fraction of opponents ("no blood for oil", war is intrinsically wrong) who mostly opposed the Afghanistan war as well, which puts them in much narrower company.

Arguments made against the Iraq war at the time (many of which are at least hinted at in this short piece by Dahlia Lithwick) included:

1. that an immediate war was not necessary because Saddam Hussein was not on the verge of doing terrible things
2. that the US was likely to face a lengthy and costly military engagement
3. that the new Iraqi government which replaced Hussein's regime wasn't likely to be a very good one (liberal, democratic, effective at maintaining peace & stability, etc.)
4. that the war would be led by the Bush administration so we'd end up with the outcomes of the Bush administration's war, not the imaginary Iraq war that you might want (this is one response to djcb's point: if you can predict in advance that things will be handled badly, then that means that the war as such is a bad idea)
5. that much of the available information about the reasons for war was coming from the Bush administration, and there were reasons to be suspicious about significant parts of the case that they were making, which cast doubt on the foundations of the whole enterprise (see also Daniel Davies on this point)

But the simplest case is just that war is a destructive, costly thing, so if you're going to get involved in one you'd better have a damn good reason and you'd better get it right. The war needs to accomplish something at least as big as its cost, which means the bigger the war the higher the standard. Iraq didn't meet that standard at the time, still less once it turned out that there were no nuclear/chemical/biological weapons. Most of the arguments were variations on that theme, demonstrating the high costs of this particular war or poking holes in the various reasons given for the war.