It seems to me that McCain's habit of invoking his POW status to explain himself might be best rebutted by pointing out all the things he SHOULD have learned from the experience, but apparently didn't. Indeed, what's obvious now is that he learned far more by having his ass handed to him in South Carolina in 2000.
I offer the following list as a brief introduction, with the hope that others will contribute their own offerings.
Because McCain was a POW, he should never have agreed to do any kind of business whatsoever with an administration that argued he had never been tortured.
Because McCain was a POW, he should know better than to own 7 or 8 or 11 (or even 2) homes when there are 250,000 homeless veterans.
Because McCain was a POW, he should not side with a strategy of preemptive war under any circumstances.
More after the break...
Because McCain was a POW, he should have found a way to sympathize with a woman (his wife) who suffered greater injuries than his own while he was incarcerated.
Because McCain was a POW, he should have known we were not (as he said) greeted as liberators in Iraq.
Because McCain was a POW, he should not have attacked former POWS and the family members of POW/MIAS still hoping to find traces of their loved ones in Vietnam.
Because McCain was a POW, he should concern himself with matters greater than congressional parking spots at airports.
Because McCain was a POW, he should have known better than to abuse power as soon as he got it.
Because McCain was a POW, he should know not to steal the stories of others who suffered privations of their own.
Because McCain was a POW, he should have a better reason for running for president than simply "it had become my ambition to be president." (Oct. 3, 2002 review of McCain's book by Janet Maslin)
Please, add your own...I will update as others come in.
[Update]
Inspired by Psilocynic:
Because McCain was a POW, he should not lash out at others as "turncoats" when he himself betrayed state secrets while under duress.
From Banjolele:
Because McCain was a POW, He should understand the needs of our veterans and not turn into Scrooge McDuck when it comes time to fund their needed benefits.
From LaughingPlanet:
Because McCain was a POW, he should denounce the swiftboat attacks on John Kerry and apologize for trotting out Bud Day to attack retired 4-star general Wes Clark.