A US-Iranian war in 1980 would have been extremely costly for both sides. The Iranians would pull all their (formerly pro-Shah) generals out of prison just to fight the Yankees, much like they did OTL against Iraq. There would be, however, a strong element of suspicion still present, since many of those generals would have been buddies with American ones in the days before the Islamic Revolution. The Iranian forces would still have a lot of their functioning hardware, since most of that got used up against Iraq, but of course US forces could pretty much trash Iranian units one-on-one. Training, skill, and experience make a great different between the two.
Iraq and the Gulf States would take the chance to resolve some claims and issues with Iran, mostly by stealing back claimed territory while the bulk of Iranian forces are held back in the defense of the motherland. Iraq's case would be trickier, since said claims are on the mainland and would be much easier to defend, so the Iraqis will be allies of opportunity to the Americans.
Morale-wise, though, the Iranians would have an edge over the Americans. The hatred of Americans due to their role in supporting the Shah is still new, and the US would have only gotten out of Vietnam only 5-6 years prior, so the memories of that debacle would be fresh on the American public's minds. The US forces will also be aware of this, so there'd be increased pressure to do better here, something the Iranian regime might take advantage of.
To be honest, the best outcome for the USA (short term) is a combination blockade and aerial/missile bombardment. Basically wreck the Iranian infrastructure and economy, tossing them back into the stone age, and leave the economy in shambles. Needless to say, this only satisfies an immediate desire for revenge, and does little long-term besides a long and painful Iranian rebuilding and creating even more deep-seated grudges against the USA.
An all-out invasion would be insane for anyone to consider. The conventional battles will be brutal, given that the US trained Iranian forces prior to the Revolution, and the Iranians can be somewhat decent on the small unit level, if lacking coordination and strategy on the higher levels. That the Revolutionary Council tossed most of Iran's generals in prison following the takeover would probably help the US in that regard, as those were the most experienced commanders Iran had. But after that, you'd have to deal with the occupation, which would be the bloodiest and most damning part of the war. The USA must take and hold some of the roughest territory possible, fighting not only against a hostile army but a potentially massive insurgency in its controlled territories. While said resistance may not be as organized as it would be today (owing to not having studied the science of insurgency warfare in the 1990s and 2000s), it would certainly be reminiscent of the darker fights in the Vietnam War. Iran in 1980 had ~40M people, and given third world demographics, that means a lot of young men, perfect for military conscription and guerrilla warfare fodder.
The Soviet Union has no love for the Iranian regime, but seeing two of its foes in a fight to the death would be an incredible opportunity. It might decide to stay on the sidelines to loudly condemn the US for its brutality, or it might start secretly selling weapons to the Iranians through Yugoslav and Afghan intermediaries, creating a bigger Afghanistan scenario for the United States to fight.