Call me naive, but...

the assurance in the last sentence of the first paragraph of the Joint Comprehensive Plan of Action between Iran and the E3/EU+3, while obviously not sufficient in itself, is not insignificant either:

Iran reaffirms that under no circumstances will Iran ever seek, develop or acquire any
nuclear weapons.

That's reaffirmed,  by the way, because Iran has asserted repeatedly that Khamenei declared as much in a fatwa, though that fatwa was allegedly never written down. Now, there it is, and again, standing alone as the third numbered provision in the document's preamble:

iii. Iran reaffirms that under no circumstances will Iran ever seek, develop or
acquire any nuclear weapons.

Leaving aside the physical obstacles to weapons development established by the treaty, how exactly is a theocracy that's declared nuclear weapons to be a sin supposed to walk back this most public of avowals?  In response to some crisis or mutual accusations of abrogation I suppose the regime could  say, "we never anticipated the mortal peril our enemies' perfidy would place us in." Or perhaps, "When dealing with Satanic enemies, subterfuge is necessary and permissible."  But when a minor power has effectively sworn an oath to the world's largest powers and the United Nations, it seems like a not-negligible --though again, far from sufficient--deterrent in itself.

The preamble is tonally interesting in other ways. It expresses an aspiration that reverses Reagan's dictum: "verify, and in so doing build trust." It seems to express the intimacy of marathon negotiators, and perhaps the beginnings of trust developed in 20 months' adherence to the interim agreement struck in November 2013 (though as Nicholas Burns points out, the mindset of Iran's negotiating team is very different from that of its powerful Revolutionary Guard, the spearhead of its adventures abroad and a major political force at home). In the first paragraph, all sides affirm that they are shifting "their" approach to the issue under negotiation (my emphasis):

The E3/EU+3 (China, France, Germany, the Russian Federation, the United Kingdom and the United States, with the High Representative of the European Union for Foreign Affairs and Security Policy) and the Islamic Republic of Iran welcome this historic Joint Comprehensive Plan of Action (JCPOA), which will ensure that Iran’s nuclear programme will be exclusively peaceful, and mark a fundamental shift in their approach to this issue. They anticipate that full implementation of this JCPOA will positively contribute to regional and international peace and security. Iran reaffirms that under no circumstances will Iran ever seek, develop or acquire any nuclear weapons.

While the proximity of "Iran's nuclear programme" to the pronoun may seem to create some ambiguity as to its antecedent, the plural possessive can only refer to the plural subject of the main clause. That's made clear in the next two paragraphs, which lay out the aspirations of the counterparties in turn:

Iran envisions that this JCPOA will allow it to move forward with an exclusively peaceful, indigenous nuclear programme, in line with scientific and economic considerations, in accordance with the JCPOA, and with a view to building confidence and encouraging international cooperation. In this context, the initial mutually determined limitations described in this JCPOA will be followed by a gradual evolution, at a reasonable pace, of Iran’s peaceful nuclear programme, including its enrichment activities, to a commercial programme for  exclusively peaceful purposes, consistent with international non-proliferation norms.

The E3/EU+3 envision that the implementation of this JCPOA will progressively allow them to gain confidence in the exclusively peaceful nature of Iran’s programme. The JCPOA reflects mutually determined parameters, consistent with practical needs, with agreed limits on the scope of Iran’s nuclear programme, including enrichment activities and R&D. The JCPOA addresses the E3/EU+3’s concerns, including through comprehensive measures providing for transparency and verification.

Note the aspirations to "build confidence" and "encourage cooperation" and the affirmation that the agreement reflects "mutually determined parameters."  That's pretty warm language for a treaty that imposes strict curbs on development and an invasive multi-decade inspection regime. It reflects a balancing of interests and of less concrete needs, characterized with a touch of gleeful cynicism this way by Ploughshares Fund president Joe Cirincione:

It is cleverly crafted so that all sides can claim victory. Iran can say with pride that its rights have been recognized, that sanctions will be lifted, and that it will not destroy a single nuclear facility.

And they will be correct. The beauty of this agreement is that Iran gets to keep its buildings and we get to take out all the furniture.

This tone-reading is irrelevant unless the deal's constraints on enrichment and inspections regime are adequate and implemented successfully. It will look pretty silly if  the deal ends in recrimination and violation. But I don't think it will, as long as Republicans and Democratic AIPAC shills fail to sabotage it -- and Khamenei doesn't kill it before it's adopted.