As a forum for diplomatic tag-team matches, I think the United Nations is great. There needs to be a place where representatives of nations can talk multi-laterally and try to solve diplomatic problems. But as soon as it arrogates to itself the powers of government (legislation, adjudication, administration), I respond as did Professor Wagstaff: "Whatever it is, I'm against it!".
One reason I think the U.N. is fatally flawed as a body for anything but diplomacy, and possibly for humanitarian work, is expressed by Australian PM John Howard, responding to Kofi Annan's assertion that the invasion of Iraq was illegal:
The problem with the United Nations - it is a wonderful body in many respects and it does great humanitarian work - is that it can only proceed at the pace of the collective willingness of the permanent members... You are seeing it now, tragically in Sudan. The body is paralysed. It is not doing much and the reason is you can't get agreement among the major powers. And people are dying, thousands of people are dying every month in Sudan.
How do you get around this kind of problem? If this was the way politics worked in any Western democratic republic, the voters wouldn't stand for it. There is no such check on members of the U.N.