Best estimates today say that Iran now has enough nuclear material to go on to the next step in nuclear bomb making. They continue to insist that they only want nuclear power reactors. You know, the cleanest available reliable power on the planet that aging hippies who watched Jane Fonda's China Syndrome too many times won't let us build in the U.S.?
Iran's afloat in oil, but it's harder to extract and refine than the oil available to many Middle Eastern nations, so it's surprisingly reasonable for them to want nuclear power reactors. Why are people scared, then?
Traditional Mutual Assured Destruction deterrence like the U.S. had with the U.S.S.R. relies on the concept that both nations desire to continue to exist, so neither will launch a nuclear attack that will end both. Iran is scary because the real rulers there are the mullahs. Iran is a theocracy, and some of them truly believe that the only way to achieve the end goal of their faith is to vaporize Israel and attack the West. How better to do that than with nuclear weapons? In short, we're afraid of Iran with nuclear weapons because we believe that they're much more likely to use them than almost any other nation on earth.
The U.S. unleashed low-power nuclear weapons to end a horrible war before we really knew the dangers and potential repercussions. We learned, and we never used them again. Iran knows the full repercussions, and doesn't care. That's scary.