As an European I would love to visit Iran. Amazing country, some friends there, direct flights... But I would lose ESTA and would need to apply for a visa before visiting US.
I feel you, this friend of mine had the same thought one year ago, he then decided that it will be ok for him to not visit the US in the next ten years then he went to Iran and had a lovely time there, he was so overwhelmed by the iranians and didn't regret anything.
One of the funniest thing he saw was this kid dancing with a donkey costume and blasting a trumpet (you know the annoying vuvuzela thing) in a mosquee, what a dangerous country.
A fantastic, cheap country where almost everybody speaks OK english, great and amazingly helpful people. And the most important thing - completely unspoiled by mass tourism. You just don't see that in the world where there is actually something to see anymore.
I gained much respect for persians and their history when doing a short trip there (Mont Damavand, Isfahan, Yazd & desert, Tehran).
Losing an ESTA for such an enriching experience? Definitely worth it. Plus when I flew through Miami last year, I had already new passport and all was OK.
A few years back, one of my ex-colleagues who needed to travel to US frequently for work was stopped right before boarding a plane because he had a layover in one of the blacklisted countries while on holiday. He didn't even know he lost his ESTA.
As an European I would love to visit Iran. Amazing country, some friends there, direct flights... But I would lose ESTA and would need to apply for a visa before visiting US.