Obviously, a solution that is agreeable to everyone is best, right?
Consider the following situation: nation A (let's call them Athens), nation B (Sparta), and nation C (Troy) all meet to try to come to an agreement on human rights. Athens is a shining beacon of democracy in the world, they even like to spread the fantastic-ness of democracy to other countries. Athens supports humanitarian intervention and human rights around the world. Sparta, on the other hand, has a bit of a problem with a minority within their territory, a problem that the Spartans have decided to deal with by denying every human right to these citizens and initiating a campaign of ethnic cleansing. The Trojans are the moderates: they support human rights, but not at the cost of national sovereignty.
Should Athens, Sparta, and Troy attempt to agree by consensus, anything they pass must meet only the lowest common denominator of the three doctrines on human rights and intervention. Therefore, any resolution that the three pass will be completely ineffectual in practice, but will allow Sparta to continue killing their own citizens; Athens can proclaim a victory in the monumental passage of a resolution on human rights by consensus (every UN diplomat gets excited by that little buzzword); and Troy can claim to be the moderator, the calm and sage-like arbiter from whose fertile mind this consensus sprang.
Everyone wins! (Well, except the Spartan minorities.)
This is the danger of consensus. If everyone agrees, there is probably something very wrong, especially in an organization like the United Nations, which, by design, includes almost every possible viewpoint on almost every possible subject.
In fact, the UN Security Council veto power, an established power of the permanent five members of the Council that I am very much against, was established so that decisions could, and would, be made without consensus. Unfortunately, the founders of the UN didn't foresee the P-5 being the very nations pushing for consensus and ignoring the plights of others. Democracy is a great thing, but when it is in almost everyone's best interests to ignore a problem, or even worse, when they are encouraged to ignore a particular problem in order to reach the diplomatic Eden that is consensus, people suffer.
I've been trying to write a coherent post about the UN for a few days now, and I will probably write more on it in the future. The UN would make great fiction - a struggle for international governance, rather than regional governance; the problems and conflicts that arise when governing a whole planet; even expansion to other planets. I think the problem with people who like the UN (myself among them), is that we just can't comprehend why other people wouldn't want what we want - a more peaceful world(s) with better leaders and better living for all. Maybe it's something about that whole having an omnipotent being/organization above you. God doesn't seem to want to share, I suppose.