The U.S. will be ok in Afghanistan as long as they don't manage to get the "occupier" label dropped on them like happened (due to our own ham-handed tactics) in Iraq. As long as they are seen as keeping the Taliban at bay, things will be smooth. The day that changes is the day for NATO & the U.S. to beat feet.
The Iranians would, in the short term, have done okay. They could put more combat power on the border than the Taliban and would have had some initial success. Their problems would have begun after the initial thrust. They lack the air power to hold down the country while the Army tries to close with the Taliban, and just as importantly, the Northern Alliance.
The Iranians also have to leave sufficient forces at home to prevent any opprotunist, be it be Saddam, the Kurds, Turks, or Americans from saying "No time like the present" and rolling into a big part of the country.
It would have, from a Western perspective, been a wonderful little war. You can more or less count on Bin Ladan & his crew to come out & fight, with a bit of luck they will get killed by brother Muslims. The Taliban gets ripped up to the point that the less crazy elements in Afghanistan can topple them. Iran's Revolutionary Guard gets torn up in an unpopular war that, with some careful goading might result in the Supreme Leader & his ilk doing the Danny Deever.
Almost too bad it didn't happen.
) The Afghans are too fragmented to accept anything else than tribal rule. The US thought they had rid Afghanistan of the Taliban last year but they popped back up.