Maybe it had something to do with the fact that he had fought the Nazis. Maybe it was because he would run toward an invading alien armada carrying nothing more than a shield, and you suddenly felt sorry for the aliens. Sure, he had the enhanced strength afforded him by the super-soldier serum, but it never felt like that process so much made a hero as . . . unleashed one.
The Captain has made it into the live-action realm a couple of times which I "highlighted" in this earlier blog entry, but, until the recently released full trailer for Captain America: The First Avenger, it hasn't felt like the hero has really received his due.
I'm not completely sold yet, but I'm getting very, very close.