John Kerry and Javad Zarif, had much to overcome. In the United States, the Republicans led by McConnell and Boehner, with sniping from McCain and Graham., joined with Netanyahu of Israel ( who was invited by the Republicans to address Congress), hoping to stop any deal. I Iran the Grand Ayatollah and conservatives made various attempts and statements, hoping to derail the talks. They seemed to have failed but they will have another opportunity, when it comes time to ratify the deal.
Iran has suffered very much from the sanctions that were imposed and they realise that the only way for relief, was to come to an agreement on their nuclear programme. Both sides were suspicious of the other after 36 years of tension and accusations. The Iranian were still bitter over the American support for the repressive regime of the Shah, whom the CIA had put on the Peacock throne,and for their support of Saddam during the 8 years of war and the Americans were still furious with the over the hostage taking and Iran's involvement with Hizbollah and with their connection to Iraq's Shia-led government and also with their support of the Houthis in Yemen and of course their seeming hatred of Israel.
The election of Barak Obama and later of Rouhani, signal a new beginning. Pres. Obama had made peace with Iran a priority and though initially they had little success with Ahmedinajahd, the Americans did not go away but decided to back diplomacy with economic sanctions and embargoes. With a newly elected, moderate government in Teheran, which signalled that they were willing to resume negotiations (the sanctions were really hurting), the talks resumed, with Russia and China, playing significant roles.
Iran has agreed to verifiable limits on their nuclear programme, inspections of their facilities and modifications to their weapons production. In return, would get tens of billions of dollars economic relief, can sell their oil ( which is crucial to their economy), and the lifting of the UN arms embargo and that the Security Council stop describing Iran's nuclear activities as "illegal".
The immediate effects would be beneficial to all parties. Tensions with Iran will be eased; cooperation in the fight against ISIL in Iraq and Syria; Iran using its influence with the Iraqis Shia government and with Assad of Syria and with the Houthis in Yemen. Iran will be no longer under the yolk of sanctions; will alleviate the suffering of the people; will sell their oil and bring in much needed revenues ( this will also mean more oil available in the world market and a drop in the price, benefitting customers, nations and people), and as a bonus the ability of the US to deal with the drug trade from Afghanistan using the Iranian corridor.
The deal when finally completed will ( as mentioned above), have to have the support of the American Congress and the Iranian Majlis. There will be opposition, especially from the friends of Israel in Congress and from Jewish lobbying groups, and in Iran from conservative elements associated with the Grand Ayatollah, but that will pass and the world can breathe a sigh of relief.
July 14, 2015, the deal has been reached. Now it has to be ratified. There will be opposition, but it will pass. Sanctions on Iran will be lifted and the people will be released from that yoke. Iran will sell its oil and consumers, individual and state will benefit. There will be cooperation on Iraq, Syria, Yemen, Afghanistan . Iran will scale its nuclear programme, reduce centrifuges and allow for un fettered inspections. It's a great day.