Oddly enough, I didn't think there was any need for a "worthy opponent" for Steve. Steve's heroism isn't defined by the baddies he fought because he's not a one-man army. His heroism resides in that he did what any decent human being would've done. Red Skull is just another generic bully, who is no different than the dude who beat the crap out of Steve at the beginning of the movie. I think that's the point. There's no real "grey" for Captain America to navigate, because the first movie is an origin movie. It's supposed to show us that Captain America fights bullies and not there to spout ideology. It's only when we've gotten to know Captain America as a good man that his highfalutin moral stance doesn't come off as grandstanding.

The first movie's theme was about Steve disliking bullies, whether they come in the form of a back alley punk or a superpowered villain, they all have one thing in common: they are stronger, thus bully people. It's very different from the second movie, where the theme has changed from fighting against bullies to figuring out what is right and understanding that "do what needs to be done" doesn't mean "ends justify the means". So, I thought Red Skull actually worked quite well as a villain, simply because character conflict wasn't the central theme of the first movie. Whereas in the second movie, Captain America had a lot more internal obstacles to over come and had to re-establish where he drew the line. (The line, of course, being Bucky. XD)