Kevin Drum links to this review of The Best Intentions, a book about Kofi Annan's years at the UN, especially the time since 2002-03, when the US explicitly rejected international law and the UN process and attacked a sovereign state that posed no threat to America.
It's telling that all of the criticisms of the US come from it's strength - the overweening pride of wealth, the cavalier militarism of the well-armed - while all the criticisms of the UN come from it's weaknesses - unable to stop the genocide in Rwanda or the atrocities in Bosnia, unable even to control the corruption of the oil-for-food program (which was dramatically overplayed, but real.)
The solution to both is as simple as it is unrealistic: the UN, or it's successor, needs several degrees of independence from it's strongest member states. We've already accepted this in principle, with the Korean War: The UN intervened against an ally of the Soviet Union to uphold the UN Charter. I'm under no illusions as to the Cold War power plays involved in 1950, but the principled argument hold up nevertheless.
Imagine, for a moment, that in the early winter of 2003, Kofi Annan was head of the UN, but instead of being powerless to stop the US invasion of Iraq, he was able to place tens of thousands of peacekeepers on the Kuwait-Iraq border. Impossible in the real world, of course. But if it had worked, wouldn't the US be better off today?
This is the point about building an independent multilateral organ of global governance - whether it be the UN or some new body. Even in the worst-case scenario for the Americans (such as the above hypothetical) America is still better off with the UN than without one.
Matthew Yglesias is one of the few writers I've seen who truly understands this, which is why I tend to link to him rather copiously. The point of the UN is not to be some kind of super-NATO. It's not to ensure that the US gets its own way in the world, every time. The UN is there to ensure that, even when America doesn't get it's way in the world, the world (including America) is still better off.
The corollary to all this is that people who complain that, yes, undemocratic regimes in the UN have just as much say as democratic regimes are missing the point, inadvertently or otherwise. The UN's role is not to represent the people we like. It's there to govern the world, and the reality of global politics means that the UN gets all of the toughest, least forgiving jobs (Darfur, Rwanda, AIDS, etc.)
The US could, if it liked, be like Japan in the 1930s and leave the UN and go it's own way, but the disaster would not be for the UN alone. Ask the Japanese how well it worked for them.