If you remember, UN was formed after the world faced two nuclear bombs, rather than the end of WWII. Though there weren’t any more nuke attacks, but neither was there any disarmament. In fact, more countries were born with there indigenous nukes and many more are yet in utero, obviously they couldn’t let it be one man hegemony.

If on one hand UN agencies like WHO has achieved a lot over the years. But others like UNHCR have not yet been successfully able to manage the huge tide of refugees at Af-Pak border. UN has over the years given rise to groups within this 192 nations big group. But still the fate of UN hangs on tenterhooks of big fat nations funding it heavily like US (~22% of its budget), and this in turn allows it to do what it feels like.
Post 1945 we witnessed long period of cold war, the gulf wars, the Indo-Pak wars, Iraq Invasion, rise of terrorism all around the world and human rights violation yet goes all around the world, sometimes I wonder the value of ICJ when "Compulsory" jurisdiction is limited to cases where both parties have agreed to submit to its decision. Millennium developmental goals are achieved far-far-from any satisfactoriness. Sometimes I wonder what the world (or rather America) was doing when Taliban was having a safe haven in Afghanistan, the truth is it was America only who built it at first place. Now on the same lines lies Somalia.
But yet it has given platform to voices which would have otherwise remain crushed, few months back when Ahmadinejad in New York was slamming America and its allies for the presence of nuclear weapons in this world, the only thing what American diplomats and those from its could do is to walk out.

In nutshell, in the past half century UN did make an impact on this world but it seems to be shedding its teeth as it grows old.