US to leave the UN?

[GermanGuy]

The UN does not have the power to force the US or any other country to implement any of their decisions. (see Kyoto or the International Court of Justice). The only thing that comes close to "forcing" power is the security council, but that can't stop a country like the US to go to war (okay, maybe it would have needed some more power to let UN inspectors look for weapons of mass destruction in Iraq...).
In economic matters the UN does have a big influence, but that only on countries that get money from Worldbank or IMF so, that should not be a problem for the US.

So the UNs problem is precisely that it is only an organization where countries "talk to each other" and not that it has to much influence.