There has been a lot of discussion about the ineffectiveness of the UN. That is the primary criticism coming from conservatives, that the UN talks and talks and makes resolutions and does not have the backbone to enforce them, instead relying on member states to provide the muscle for any operation the UN has resolved is necessary. There is an easy way to get around that. Introducing: The United Nations Strike Force.
The United Nations Strike Force (UNSF) would be responsible for the "first response" of any UN legislation that requires military action. In such areas as Darfur, Bosnia, and anywhere around the world where the UN has decided it necessary to step in and take a side for world peace and prosperity. It is a small, elite military unit (comparable to the Navy Seals, Army Rangers, Green Barets, etc.) that operate under the expressed juristiction of the UN. It is a small but well-equiped unit, with all the latest in warfare technology. A hybrid force of aerial, sea-based, and land forces that could work together in a fast-paced, high risk environment. The actual size of the unit would depend on the mission involved and the particulars of the creation of the unit by the UN, but I would imagine somewhere on the order of 10,000-20,000. Obviously some parts of a military would have to be foregone (aircraft carriers, for instance, require too many personnel to be operated effectively), but they could be surmounted by innovations in technology and planning (using the Joint Strike Fighter would enable vertical take-offs, putting these jets on smaller ships without the usual crew needed on a carrier).
The personnel involved would come from the member nations' militaries. However, they would be on a multi-year loan from their country to the UN. While they are serving the UN, they are UN Soldiers, not American/Canadian/French/etc. soldiers helping with a UN effort. Their loyalties lie with protecting the UN Charter and the decisions of the Security Council. They are UN Soldiers while they are there. The council would, of course, do their best to avoid sending troops to areas of the world where there would be a conflict of interest. So, any Muslim troops would not have been forced to fight against Saddam Hussein (but if they wanted to, no problem).
This force would not be the size of a regular standing army, and once they had cleared the way, removed major obstacles, then regular, member-sponsored peacekeepers would take their place. This would be a dangerous position to hold, as the soldiers would be working in some very hostile environments against a myriad of enemies. But they would be defending global peace, freedom, and rights, and that certainly is a just cause to be working towards.
Perhaps the most important part of this is that it would force countries to think twice before defying the UN's resolutions in the future. No longer could they scoff at sanctions, demands for cessation of activities, requests for entry to genocidal areas. For the UN would have a powerful muscle to strike with, and force them to capitulate to the UN. Used correctly, this could be a good tool to wrangle with despots the world over, who know that they can blow off the UN and not have to face serious consequences. That would change, and we could work quickly and effectively towards limiting conflict and establishing peace across the globe.