The problem is the simple. Unlike North Korea, Iran is supporting a spreading culture and idealogy. If Iran gets nuclear weapons, any hope of stopping the spread of radical islam is lost.
Both North Korea now and Iranian leadership soon will be secure in their positions, since it is known they are crazy enough to use nuclear weapons if they think they are going to lose power, and not care about the consequences to their own people.
So, what we would end up with is a government immune to sanctions and threats of violence still supporting and funding the terrorist culture. It could use influence with impunity in the middle east to steer elections and governments, without fear of attack.
In the modern world, having nuclear weapons is like having an inpenetrable forcefield around your country as long as you don't use them. This would mean the middle east nations would stop moving towards democracy and moderation, and turn right back and head towards fanaticism.