The Problem with the UN, to the extent that it is anything more than a forum for discussion, is that (a) it expects countries with conflicting national interested to cooperate, and (b) it expects nations to surrender large portions of their sovereignty to an organization that consists of people who do not share a common culture, common goals, or a common political system. In short, it is an attempt to establish world government.
Idiots in the conservative movement are convinced that the only problem is the part about a common political system, or more specifically, that not all countries are like us. Cal Thomas approvingly cites a suggestion that we leave the UN and create a second UN-like organizations that only lets democracies join.
The problems with this are legion: (a) it will still destroy sovereignty, (b) certain non-democracies like China or semi-democracies like Russia would not get to join in, which they would see as provocation and which would hurt our relations with these countries (new Cold War, anyone?) (c) It could make the "new UN" just a fig leaf for the powerful countries in the world to trample over non-member countries with the excuse that those countries are not democratic enough.
I'm more in line with what Michael Reagan appears to be suggesting: end the UN and don't replace it.
That is all.