I've been hearing this crap my entire adult life. I think it all started back when they nationalized their oil resources and the "hostage crisis" back in 79 just put more fuel on the fire. When I ask myself "What threat Iran poses to this country?", I need only consult history.
Question: When has Iran, in modern history taken aggressive action toward another nation?
Answer: Never!

Question: If Iran did develop nuclear weapons would their capacity be a serious threat anytime in the next two decades?
Answer: Slim to none. If they did develop weapons the would be like a grain of sand on the beach compared to those that already exist.

Question: What if Iran took aggressive actions against another country? Take your pick, conventional or nuclear weapons.
Answer: There would be immediate retaliation. Depending on the level of aggression it could be anything from severe conventional strikes to total annihilation.

Iran is currently surrounded by US forces. We are, with the assistance of most of the developed world enforcing crippling economic sanctions, as we had with Iraq, that primarily harm the population not those in political power.

It might seem to be a trite soundbite but "It's time we stopped pissing in other peoples sandboxes".
As a nation we have been doing our best to generate enemies since we invaded this country.
It's far past time we stop this shit and start living on the planet as equals.