The interim nuclear accord between Iran and the six world powers is a significant accomplishment by any measure. The agreement is the product of years of co-ordinated, sustained diplomacy, mostly involving the US and its principal European partners.
Success also reflects the sophisticated use of diplomacy against the backdrop of economic sanctions and the credible threat of military force. In addition, the accord demonstrates the ability of the new Iranian government of President Hassan Rouhani to compromise and its desire to get recognition at home for what it receives in return.
The interim agreement itself places meaningful constraints on several dimensions of Iran's nuclear programme in exchange for providing Iran with a degree of financial relief from existing economic sanctions. The accord, better understood as a ceiling than a freeze, also establishes a level of inspections that is far more intrusive than what has existed. The net result will be to slow the pace of Iran's progress towards putting into place the many elements of a nuclear weapons capability, in the process increasing the time and warning the world would have between any Iranian decision to produce one or more nuclear weapons and when it would actually achieve that goal.
What the interim agreement does not do is dismantle important aspects of Iran's nuclear capacity or potential. This is an agreement that does limited things for a limited time, no more and no less.
Those who are opposing the interim accord for what it does not do are asking too much. The measure of any diplomatic agreement cannot be the possible versus the ideal but rather the possible versus the realistic alternatives, in this case either living with an Iranian nuclear weapons capability that would lead others in the already unstable Middle East to follow suit or launching a preventive military strike without knowing in advance what it would accomplish or set in motion. This interim pact is far preferable to either alternative.
Also not realistic are arguments suggesting that since sanctions are working, there is no reason to enter into an agreement that falls short of the objective of getting Iran out of the nuclear business altogether. International support for sanctions would collapse long before that goal was in sight. And this or any Iranian government would opt for confrontation over negotiation if humiliation was the certain outcome of the latter.
The real question to be considered surrounds not the interim accord just completed but the follow-on or "comprehensive" agreement to come. The announced aim is to finish negotiating and begin implementing such a pact within a year. The incentive for Iran is obvious: the agreed upon wording promises the end of all nuclear-related sanctions (but presumably not those sanctions linked to other aspects of Iran's behaviour that are found objectionable).