First of all, a deal with Iran on their nukes that is successful in depriving Iran of nuclear weapons is likely to be a bigger version of the Syria deal. That is, an agreement on nuclear weapons leaves Iran free to carry out a hostile foreign policy at odds with our interests:
The groundbreaking dialogue between Iran and the U.S. has raised speculation of further advances to ease their 34-year diplomatic estrangement. But the two countries have overlapping interests across the Middle East and beyond that are sharply at odds.
So if we are less able to battle Iran (and threaten harsher actions if Iran goes too far), Iran will have more abilities to push their agendas in Syria, Afghanistan, Iraq, Israel, Lebanon, Gaza, and the Persian Gulf.
Further, President Obama has made it less likely that we could get a real deal on Iran's nuclear weapons. Remember, we flipped Libya and got them to give up their nuclear and chemical weapons programs after Khadaffi got scared of following Saddam into the prison, trial, and dirt nap route.
But then in 2011, President Obama led a war against Khadaffi that allowed rebels to shoot Khadaffi. Without WMD as leverage and without Russia or China shielding them at the UN, Khadaffi could only hope that the NATO alliance would fracture at the duration and cost of the war. (I think Khadaffi erred in not lunging at Benghazi early in NATO's intervention to get into the city regardless of the cost marching up the road. Only human shields could save Khadaffi at that point.)
So Iran can't trust us to abide by an agreement that Iran is willing to complete (as unlikely as that is already) based on our recent history.
I fear that President Obama is willing to agree to a deal that Iran pretends to end their nuclear weapons programs (which they deny having, in any case). He'll pretend it is a real agreement and with any luck it won't be shown to be a sham until after he has left office in 2017.
Meanwhile, if an agreement is signed, Iran will continue to work on their nuclear weapons program. Perhaps they really will suspend their drive to get nuclear materials while they get their long-range ballistic missile program caught up with the nuclear work. After all, having a nuclear device without the ability to launch it is the worst of both worlds--bomb bait that proves Iran lied without the ability to threaten nuclear use to stop the attacks.
In the end, it's the regime we should be worried about. Iran under the mullahs is a threat regardless of whether they have nukes, or not. And a non-nutball regime with nukes would be less of a threat, in the worst case. I know, the nuclear program is popular in Iran. But in a non-nutball Iran, the people might think that spending on other things is worth more than having nukes regardless of the cost.
My preference to solving the Iran problem has always been regime change first. A bombing campaign has always been my fall back position to buy time to actually seriously attempt to change the regime.
I remain disappointed that the 2009 Green Revolution was abandoned by President Obama in a futile attempt to extend and open hand to Iran's mullahs. And I remain disappointed that after a decade, our intelligence services haven't been able to figure out how to spark a revolt in Iran. (Unless the Green Revolution was that event, of course.)