What are the key differences between between Saudi Arabia and Iran?

Iran and Saudi Arabia are grappling with some fundamental differences which have been fortified over time. There is an apparent Cold War which although has little potential into an armed conflict yet is powerful enough to keep the two on the boil for time to come.
Key irritants in relations:

• Sunni dominated Saudi Arabia and Shia dominated Iran share grave rivalry rooted in both geostrategic and religious fronts.
• It was the Iranian revolution in 1979 with its loud anti-US rhetoric which was seen as a two-pronged danger by the monarchies of Arabian Peninsula. Arabs were, in addition, a staunch financial backup for Saddam Hussain during his war with Iran.
• Further, the termination of diplomatic ties between the two in January 2016 after Saudi Arabia’s embassy and the consulate was stormed by Iranians in retaliation to the execution of a known Shiite cleric, following the Nuclear Deal in 2015 between Iran and P5+1. Latter was seen as a step towards the end of international isolation of Tehran.
• The recent aggressive mount up in tensions was over Qatar as all diplomatic ties were snapped by Saudis and all Sunni allies after accusing it of supporting extremism. Also, the resignation of Saudi backed PM of Lebanon Saad Hariri has fuelled the fire as latter blamed Iran for its increasing grip via Hezbollah.

Thus the historic adversaries are always mired in differences and the rivalry has become the organizing principle for alliances of the middle-east. Despite the heat, a larger conflict is unlikely.