The UN is a tiger with no teeth. I was reading news coverage today of the crisis in Lebanon. While world opinion is firmly in the cease-fire camp, and the UN argues persuasively for the need for a sustainable peace, they really can't do much.
The problem is that the UN has only two weapons in its arsenal: economic penalties and world disapproval. We can discount the 2nd weapon immediately -- because really few nations care whether the UN is unhappy with them. The sanctions are real, and can cause harm, but are often ignored or used as a political tool to stir up resentment and dissent. The sanctions are also difficult to enforce.
The UN needs a way to enforce its decisions, to impose its will. Member nations should not be able to pick and choose (like the US does on a regular basis) when to follow UN dictates, and when to ignore them. Indeed, no nation should be able to opt out of UN membership, or be excluded from membership. We need global representation, and global commitment to resolve global issues. For too long, the superpowers have had their way, been the biggest bullies on the block.
I am not sure how all of this can be accomplished. But I see problems in our future, like severe drinking water shortages, or global warming, that need a strong global organization to reach any kind of solution.