Well for this you need to know the history of Iran's nuclear program and what Iran wanted from it. In short, Iran has achieved most of its objectives (though not all). For Iran the nuclear program was to be bulwark against hegemony of Western powers. Iran has been suspicious of Western powers ambitions since at least 16th century. Not only in military dimension, but also and of equal importance in economic sphere.
To break this hegemony, Iran needed a tool that could apply to deter the Western powers from trying to dominate Iran. One way would have been for Iran to go full nuclear. This would have guaranteed Iran's military security but at the cost of economic security. The other way was for Iran to basically stand in the middle, letting the West be perpetually afraid of Iran going nuclear and through this extract security both in military and economic spheres. Iran chose the second option. Still this deal does mean it can not go down the drain as other deals did, and it does not mean Iran will not go fully nuclear in future depending on different circumstances.
As long as the technical capability exists, the threat is always there. What this deal does, puts the pressure on West. Now West has two options. Either it will have to reach a strategic understanding with Iran, as it did with China in 1972 and accepts Iran as a sovereign and independent power with legitimate interests not necessarily in line with West. Or the West refuses to reach to this understanding and the deal will eventually unravel in future, in a slow motion fashion. The onus now is on the West. It will be Obama and his Western counter-parts who will have to fight an up-hill battle to convince their public, Israel and political structure of their countries of a deal. Iran will just enjoy the show now.