I think you answered your own question. It's a lot easier to prevent a country from developing nuclear weapons that it is to confront one that already has them.
Also, different countries and different geopolitical situations. Iran has a few allies among the superpowers (Russia, China), but they aren't very strong allies. NK has pretty strong backing by the Chinese and the US also has to deal with it's allies in Japan, Australia, South Korea, etc.
It's much easier to play hardball with a country when no one is really opposed to you doing so.
I think there is also the issue that Iran is more likely to fight asymmetrically than North Korea would. I imagine that North Korea has all their nuclear weapons mounted on ballistic missiles. Iran on the other hand probably support an asymmetrical war where nuclear weapons are brought into the country by "stateless" terrorists and then deny involvement.
I would also venture that Iran, being a "non-secular" government where the more fundamentalist members see US influence as a threat to their religion and way of life, is more likely to foster radicalists that would try to enter the US with a suitcase bomb. North Korea has completely isolated itself and its people from US influence. Iran on the other hand has "porous" borders that permit western media and western culture to permeate. The permeation of western culture is seen by some as a threat to be stopped.
Edit: can the downvoter, please explain their down vote?
Also, different countries and different geopolitical situations. Iran has a few allies among the superpowers (Russia, China), but they aren't very strong allies. NK has pretty strong backing by the Chinese and the US also has to deal with it's allies in Japan, Australia, South Korea, etc.
It's much easier to play hardball with a country when no one is really opposed to you doing so.