From Philip Giraldi at Campaign for Liberty:

First and foremost is the fact that Iran does not and realistically speaking cannot pose a threat against the United States....

Second, the United States can't afford another war, most particularly a "war of choice." ...

Third, no war goes according to plan and there are always unintended consequences....

Fourth, attacking Iran after invasions of Afghanistan and Iraq would send the message that the United States is pursuing a policy of incremental regime change for the entire Muslim world....

Fifth, war against Iran would complete the transformation of America into a nation dominated by war.

Read the full article for elaboration on each point.  Giraldi is absolutely right here; I can only pray he is less right in his confident prediction that this unfortunate war is likely to happen.