I worked in the UN (in the ILO to be exact) last summer and saw really how it works. The thing is: the UN includes much more than just to keep peace on earth. The whole system is extremely complicated and enormous. I was myself working with workers' rights and I do think that the work people do there is important and needed. I though have been disappointed too about results, and I blame countries and their leaders for that, not the UN.
There is also these days a cold fact surrounding us: money equals power. Countries like China for instance do have an important role in this world due to their economical growth. Here the UN can't really do anything. Basically China could do almost whatever they want to do, no one will really interfere.