MeFi helpfully links to this interesting pro/con summary of reasons to invade Iraq.
While the summary isn't intended to represent a single, pervasive argument by either side, I note this paradox: The pro side claims that Iraq's milirary is strong, therefore Iraq is a threat, and yet says it's weak, to add confidence to predictions of an easy victory. The "con" side makes opposite contradictions--Saddam's military is weak, so it's little threat, but it is strong and so might prove much more difficult than the hawks imagine.
However, I think it's much more plausible to postulate that Saddam's military could be weak enough not to be much of an offensive threat while still being a potent defensive one. As the Germans retreated from France, for example, they demonstrated that, despite Hitler's orders they do so, they could not mount much of a counterattack against the relentless two-front Allied assault. The Wehrmacht did, however, prove to be difficult to root out, especially when it chose to stand and fight in cities.
Also, I can't stress enough that invasion vs doing nothing is a false choice. Iraq certainly is a rogue state and a threat and needs to be dealt with, but the burden of proof is on those who say that a military takeover is the only way to deal with it.