It possible negotiations do not succeed. It is also possible that common interests are found and agreements are reached that include Iran continuing peaceful nuclear development, abandoning any possibility of creating nuclear weapons with full verification procedures agreed upon, and playing a more constructive role solving problems in the Middle East, including the removal of all chemical weapons in Syria. 

Similarly, a diplomatic success would include the United States, the European Union and the United Nations phasing out and ending economic sanctions based on mutual agreements and ultimately normalizing relations in ways that promote mutually beneficial trade.

The skeptics should be skeptical; words must be matched by deeds; promises must be fulfilled by action; this is always true of diplomacy and remains true today.

It is positive that Rouhani has reached out, and it is positive that Obama and Kerry have reached back. So, it is good news that the presidents of the United States and Iran have spoken, that diplomatic talks have begun, and that there is a possibility for progress that will now be actively explored by both nations.