I almost never post anything political here, but now I find it necessary for the sake of future reference.
I took some classes in college about warfare and politics, and read books about the American experience in Vietnam. Long story short, in the course of my studies I discovered some principles of guerrilla warfare that are applicable to Iraq and Afghanistan.
Similarities: In Vietnam there was a large, conventional military (U.S.) with a supported, ineffective puppet government, South Vietnam (SV). These forces were fighting a smaller, weaker guerrilla campaign (VC) with North Vietnamese support (DRV). All of these characteristics define the Iraq and Afghanistan conflicts.
Differences: Vietnam is a highly homogenous nation, whereas Iraq and Afghanistan are highly fractured nations. Therefore Vietnam was primarily fought over political distinctions (and disenchantment with endemic corruption in SV). However, Iraq is much more defined as a religious conflict with tribal aspects, and Afghanistan is also highly tribal just as Afghani society is tribal in nature.
First, pulling out of Iraq is a mistake if anyone wants a stable Iraq. Saddam was enforcing stability in Iraq through a combination of very savvy balance of power dynamics between the various rival factions, and most relevantly, through ruthless brutality against his enemies. He was a real dictator with a widespread and powerful intelligence agency and he didn't tolerate dissent.
Ultimately, it was his power that maintained stability in Iraq. Now that Saddam has been killed, the US is providing stability through sheer force. If the US pulls out, the best case is that Shi'ite groups take control of the country and simply dominate. The worst case is the Sunni south revolts and they have a civil war. Kurdistan is another possible civil war fault line. I have no idea how these three countries (in reality they are three countries) are going to stay politically united without a strongman to keep them together. My bet is that a strongman is going to emerge and simply take over when the US leaves.
Now a positive aspect is that if the US leaves, then Al Qaida and many of the int'l fighters lose their reason to stay, so they pull out and stop destabilizing the nation. However, it's possible that these Sunni fighters will stay to topple any Shi'ite government. You can similarly expect the resurgent Iran to want a piece of the political pie, and to insert Shia agents to strengthen any Shia government that might emerge.
The Sunnis have been historically dominant though (Hussein was Sunni), so they will not take this one sitting down.
To be honest, I don't know what most people want to happen in Iraq. Do Americans not care if thousands or more Iraqis die from the ensuing chaos of an American withdrawal? Do they simply not realize what's going to happen? I'm inclined towards the second, with an unconscious helping of the first.
Let me start by disillusioning anyone of the following notion. Iraqis dying is not a problem fixed by removing the American military. Yes, we have caused this problem by removing Hussein, for the reasons I stated above. However, there must be a strong central government to take over before America can withdrawal. Otherwise, many, many more people will die. I'm not in a good position to judge when the Iraqi government is strong enough to take over, but it's my opinion that we haven't reached that point yet. Of course, Iraq is kicking us out now, so legally we have no choice. Let's hope that they are ready to take over.
The biggest different between Iraq now and Vietnam is that I am not aware of any external nations at war with Iraq. As far as I know, there isn't any RVN army waiting to march in as soon as America pulls out. So that means their state failure isn't guaranteed in my opinion. But it's definitely a strong possibility.
A lot of this calculus applies to Afghanistan as well. An American withdrawal will mean civil war, and possibly a Taliban takeover. The Afghani government is corrupt and generally unstable. They do not have a competent military. I don't know what Americans think about Afghanistan, but an American withdrawal will mean civil war in Afghanistan, which the Taliban might win. Many people will die.
P.S. I wrote this really quickly and without going into much depth, so I've probably mis-stated a bunch of things. I will correct these points as they are raised to me.