Rise of the Diplomat Shaykh
Rowhani's political orientation is not primarily in the reformist camp, however, and before we call it a defeat for Supreme Leader Ali Khamene'i, we should note that the Council of Guardians, the clerical vetting body, was not afraid to disqualify candidates whom they perceived as a threat to the system. Rowhani has actually been close to pragmatist former president Ali Akbar Hashemi Rafsanjani, and is known primarily for his technocratic approach to foreign policy and security issues. As Iran's chief nuclear negotiator in 2003, he was seen more as Khamene'i's agent than Khatami's. As far as I can tell, he has advocated reformist domestic policies for only a few weeks.
The policies of Mahmood Ahmadinejad have been cast as a rejection of both Rafsanjani and Khatami, his two immediate predecessors, for the past eight years have substituted populism and and ideological hard line for pragmatism and business-friendly technocratic economic management, as well as obviously gone against Khatami's openness on social policy. In 2009, we saw unity between the Rafsanjani and Khatami political tendencies, and this has clearly continued to the present.
If he does become president, and despite his lead here there is no guarantee he would win a second round if there is one, Rowhani would probably move to keep his highest-profile promise of freeing political prisoners, and Khamene'i would probably acquiesce in the freeing of most if not all. I expect, however, that Rowhani's energies would be primarily dedicated to foreign policy and economic management rather than social and constitutional reform. So again, while there is a moral victory in all those reformists who have refused to accept repression and voted while daring the regime to ignore their votes, an actual Rowhani presidency might not significantly advance their objectives even independent of the vetoes of policies by more conservative organs of government which we saw under Khatami.