Personally, I believe it to be a much more serious situation than mainstream opinion.
I believe that Iran acquiring nuclear weapons is as large as any crisis humankind has ever faced during its existence.
I will explain why:
#1 The immediate threat to our allies - "Mutual Assured Destruction" (MAD) is why we made it through the Cold War and are alive today. MAD worked great for us, and continues to work great for many nuclear armed Nations around the world. So far, all Nations who have nuclear weapons have a primary common desire which is "Self Preservation".
The "Supreme Leader" and those around him who actually control Iran are motivated more by religious ideology than by anything else. For the first time, those in control of nuclear weapons would be motivated more by religion than by self preservation or by ideologies that are historically compatible with MAD.
I would compare this to placing all of our nuclear weapons under the control of the Westboro Baptist Church, or even a less fanatical group, so long as it cares more about Religion than Life.
There is a fair chance that Iran's leaders would not seek to martyr Iran by destroying Israel. But there is also a fair chance they would.
#2 Immunity - Once Iran moves past the beginner stages and has multiple bombs with a good delivery system, they become impervious from us or any other Nation that would seek to keep them in check. For example: Iran could attack another Arab Country that is not our strong ally. We would no sooner interfere than we did when Russia attacked Georgia. We would not be willing to place our troops or our Nation in harms way to protect a Nation that is not our strong ally.
#3 Immunity stage 2 - Once Iran has a nuclear deterrent, we will not attempt to stop them or invade them for any reasons short of attacking a NATO nation or Israel. There would be nothing to stop Iran from continuing its nuclear program and entering an arms race. Who can say for certain that they will not end up with hundreds, or even thousands of nuclear weapons?
Thousands is doubtful, but what about 50 or 150 over the next 40 years? What would possibly prevent this once they get started?
#4 Eventual Direct Threat - At some point Iran will have a delivery system capable of delivering a direct nuclear payload to the United States. Regardless of how you view the issue in it's entirety, a new Cold War with Iran is just simply not a good thing. Once they have a couple of bombs, we will not be able to stop them from eventually reaching this point.
#5 Iran's ability to control their nuclear weapons - Every single County that poses nuclear weapons is at constant risk that they could fall into the wrong hands. Even our own country. Iran has very deep and very strong under currents of extremism. Even if their Supreme Leader wants them to remain secure and under lock and key, (which is debatable in itself), can he guarantee that 1 bomb or even some bomb material would never slip into the wrong hands?
#6 Iran's possible desire to deliberately allow them into extremist hands - Can we be sure that the Supreme Leader would not allow enough material for a bomb to slip into the hands of an extremist faction and wind up detonating in Israel, The Unites States, or another ally? I know some of you will say, "No, because we can trace its signature and know it came from Iran and we'd destroy them". However, only nuclear material that is already documented can be traced. And assuming it is traceable, you are depending on a variation of MAD. Iran may or may not believe that we would destroy them if they were attempting to say the loss of nuclear material was an accident. Especially after watching the chaos surrounding our claims of chemical weapons in Syria. Misinformation is a powerful thing. But the real issue is not whether it would work, but whether Iran might think it would work.
#7 I want one too! When Iran has nuclear weapons, everyone in the Middle East will want nuclear weapons. Israel will want more. All Arab and Muslim Nations will want the deterrent as well as the power of having nuclear weapons. Once Iran has nuclear weapons, it is only a matter of time before more Nations have them too. Eventually, someone will use them. Eventually, someone will lose control of them to extremists.
#8 Nuclear close calls and accidents - Even if Iran proves to be compatible with MAD, the more nuclear armed nations we have in the world equals the higher chance of having an accident occur that leads to full out Nuclear War and the end of life as we know it.
Read these links to see how the world almost ended multiple times:
Key Issues: Nuclear Weapons: Issues: Accidents: 20 Mishaps that Might Have Caused Nuclear War
The nuclear weapons of today dwarf the atomic bombs we dropped on Japan. Just 1 detonation in the right place with the right weather conditions could be catastrophic for the environment and the food chain. It would only take 1 detonation to collapse our own economy. Would only take a dozen to send us back to the stone age with town sized skirmishes over food and resources.
Yet we have thousands.
Take a look at just the documented arsenals of the world:
List of states with nuclear weapons - Wikipedia, the free encyclopedia
Pakistan has up to an estimated 120 nuclear bombs. This is why we give them so much money. They are on the verge of losing control and we must make sure they stay in control.
Do we want to be giving Iran billions each year too?
Currently Pakistan, India, North Korea, and Israel do not have long distance delivery systems. This will change in time. Just as it will change for Iran, whether it gets the bomb or not.