It's interesting to hear moderates and even liberals mimic many of the same talking points as right-wing conservatives when the subject of the United Nations is broached.
Meanwhile, NATO continues to grow and expand despite its original purpose having long ago expired, and few bother to question why.

It seems obvious that one organization is getting the short-end of the stick, and I'd like to set the record straight.

Quick Background

  Granted, the two are very different organizations that were created for very different purposes, but since the end of the Cold War their missions have sometimes overlapped.

   NATO was a strictly military organization designed according to first NATO Secretary General, Lord Ismay, "to keep the Russians out, the Americans in, and the Germans down."
  It's creation was not without controversy, as witnessed by Iceland's anti-NATO riots. It's also worth noting that the Warsaw Pact was only created after NATO rejected a Soviet request to join the organization.
  According to Russia in the 1990 treaty "the Americans promised that NATO wouldn't move beyond the boundaries of Germany after the Cold War." Originally only 12 members, NATO has expanded to include 28 countries.

 The purpose of the UN is much more broad and far-reaching. It's reason to exist is to  prevent international conflict, although it has extensive humanitarian operations.
   Originally 51 member states, it has expanded to 193.

NATO engaged in no military operations during the Cold War, but several since.
  Most of the UN's peacekeeping missions have happened since the end of the Cold War.

Flashy vs. Effective

  People know about the UN's failures because when they happen lots of people get killed, and that makes headlines.
  Generally people have no idea about UN successes because when they happen no one gets killed, and thus there are no headlines. Many critics of the UN are not even aware that the UN has had successes because of this news bias.
  Some of the fiercest criticism of the UN happened during its refusal to approve the disastrous 2003 invasion of Iraq, and is a big reason why Republicans retain a negative opinion of the organization.

  Meanwhile, when NATO reacts people always get killed and yet they are considered successes for that very virtue. Very seldom do conflicts that persist later on get attributed to NATO in the news media.

  NATO picks and chooses when to intervene. The UN goes in wherever it is needed.
NATO has near unlimited resources to call upon. The UN peacekeeping missions are usually bare-bones operations into fluid situations where the belligerents have been fighting for decades.
   Most critics of the UN point out that its peacekeeping forces are usually too small to be effective. Many of the same critics also declare their opposition of the UN because they are the instrument of "One World Government".

NATO's Record

Bosnia
    NATO began enforcing a no-fly zone over Bosnia on 12 April 1993, and later enforced an arms embargo of Serbia.
   On 11 April 1994, it bombed Bosnia for the first time. This resulted in U.N. peacekeepers being taken hostage.
  After the Srebrenica massacre, NATO carried out a two week retaliatory bombing campaign. How much the bombing led to the end of the conflict is debatable since the front lines of the war were largely stalemated from 1993 until Croatian military success brought the Serbs to the peace table.
   After the war ended, NATO carried out a UN-mandated 1 year peacekeeping mission.
  Generally the whole operation was a success.

Kosovo
   In order to stop Serbian-led crackdown on Albanian civilians in Kosovo, after UN negotiations broke down, NATO bombed Serbia for 78 days. This led to around 500 civilian casualties.
  There were around 1 million refugees from the war, at least half of them fled the bombing.
   The bombing led directly to the end of the conflict, although NATO troops are still there for security reasons 15 years later.

Afghanistan
   The September 11th attacks in the United States caused NATO to invoke Article 5 of the NATO Charter for the first and only time in the organization's history.
   Although the ISAF isn't technically the same as NATO, in reality this is a NATO war.
  Counting our Afghan allies, we've suffered around 15,000 killed and 25,000 wounded. Plus there has been around 20,000 civilians killed.
   It is generally understood that we will leave Afghanistan with the mission still not accomplished, and our Afghan allies unable to finish the job.
   By most measures, this is shaping up to be a NATO disaster.

Libya
   With Gaddafi's forces threatening to rally and defeat the rebels in the 2011 civil war, NATO intervened and turned the tide in the war.
  Militarily it was a success. However, politically Libya has become a mess. The government is too weak to govern the entire nation. Gaddafi loyalists and al-Qaeda linked organization continue to operate.

UN's Record

   The record of the UN since the end of the Cold War is simply too extensive to list here. Most of the UN's activities and budget is dedicated to development and relief efforts.
   Organizations that operate under the UN include UNICEF, WHO, WFP, UNHCR, UNEP, and FAO, just to name a few. United Nation organizations have won Nobel Peace Prizes seven times.

  Since NATO has nothing to compare these functions to, I will restrict comparisons to peacekeeping.
   That, of course means we are also going to ignore the fact that a significant percentage of wars come to negotiated conclusions in the UN.

UN's Failures

  The failure of the UN to react to the Rwandan Genocide was disgraceful, although it is extremely doubtful that the poorly armed force could have stopped it.
   This was almost immediately followed up by the UN's failure in Bosnia.
  This was preceeded by the epic failure in Somalia.

   It was a very bad two years for the UN by any measure. Although it seems crazy to consider that failing to stop people from killing other people is somehow worse than going out and killing them yourself.
 The news media largely hasn't reported about UN activities since then, or before then. If you haven't been watching, you might be incline to believe that they haven't been doing anything.

UN's Successes

  Since the fall of the Berlin Wall, the UN has been engaged in over 50 peacekeeping missions. 13 of those are ongoing. You are probably only aware of a handful of these.
   In many of them the UN peacekeepers were simply ineffective. However, in many others they were crucial.

Here's a partial list of successful missions:

Namibia 1989-1990
Nicaragua and El Salvador 1989-1992
Cambodia 1992-1993
Mozambique 1992-1994
Sierra Leone 1999-2005
East Timor 1999-2002

  A 2005 study by the RAND corporation showed the UN was successful in 2 out of 3 peacekeeping missions.
  Furthermore, the UN's nation-building programs brought peace seven out of eight times, while the nation-building efforts of America only brought peace in four out of eight.

   The same year the Human Security Report found a global decline in armed conflict since the end of the Cold War and that UN efforts were one of the major reasons.

Polls show that while most Americans think the UN has done a poor job, they are overwhelmingly convinced that the organization is necessary.

"There was never a good war, or a bad peace."
- Benjamin Franklin