1) Growing instability in Iran that threatens the security of the country's nuclear installations and risks the loss of material to forces hostile to Russia.
2) A change in regime in Iran that becomes much more hostile to Russia. At present Iran has never encouraged Islamic extremism in Russia and worked to stabilize Central Asia. A different government in Tehran that starts talking about liberating oppressed co-religionists from the heights of the Caucasus mountains to the northern Volga is a different story altogether.
3) Direct, immediate compensation to Russia of the accounts that would be lost should Russia agree to sanctions. For instance, would Saudi Arabia realistically offer to purchase the equivalent of what Iran wants to buy from Russia's arms exporters?
4) Russia benefits from the current Western sanctions regime on Iran because it prevents Iran's formidable energy reserves from being used as market pressure on Russia as a supplier to Europe. If Moscow isn't particularly worried about Iran's ability to safeguard its nuclear program, it isn't going to be in a hurry to facilitate the U.S. settling its differences with Tehran in order for Iran to then be used to erode Russia's market share. Some delicate way, then, of suggesting that most of Iran's energy should head eastward--particularly to a hungry Indian market that Iran is best poised to supply?
Agreed. The US will have to make major concessions to Russia in Eurasia (not symbolic ones like the one on European GBI) to get cooperation on Iran.