The problem isn't the UN, it's the UN's members. The UN was purposely made the way it is, that being not have a standing army(the only way the UN can enforce any decree). With a standing army comes increased costs(must be payed by the members). Then you have to consider what/when/where/why/who that army would be used for/against. What kind of mess would we have been in with an "effective"(by the definition being put forth here) UN during the Cold War?
Not being an expert on the inner-workings of the UN, I'm going out on a limb here, but go I will. IMO, the Security Council is likely the problem with the UN. During the Cold War, the USSR and China were philosophically opposed to the US, France, Britain, and the other members(or are these the only permament members?). Anyway, this schism severely crippled the effectiveness of the UN, although some very important and effective actions were still able to be implemented(Korea, Cuba Missile Crisis, Peace Keeping missions in various conflicts, etc). Now that the Cold War is over, and after many years of a wearing down of dedication of the various SC members(I remember the US complaing about the UN's communist tendencies during the Cold War, I'm sure others felt similar things, and it wouldn't surprise me if the USSR and China complained about the capitalist tendencies), the SC seems to still not be dedicated to the idea behind the UN. Now those same members are more concerned in their own interest, which continues to work against the effectiveness of the UN. They seem to be involved only because it's tradition(similar to how some people attend church, they just do), rather than try to make the UN concept work. After years of stagnation at the top, most of the other members probably don't expect much from the UN either(disillusionment).
Should the UN be abolished then? I dunno, if it is, it needs to be replaced with something similar. What I'd like to see, and why it's not being done boggles my mind, is for the US(and whoever else on the SC that wants to join them) become more active in the UN. The US(and it's citizens) states repeatedly it's desire to spread Democracy/freedom across the globe, why not use an organization where practically all the nations in the world gather on a regular basis? Why not be an example instead of constantly complaining and only show up when it's in your best interest?