I was thinking about it, and I think the difference might be that Peyton Manning is a student of the game. By that, I mean that Peyton's success is due to knowing how to read the defense, knowing who's going to be where and when, knowing when to trust his receivers and when to play it safe, etc.
Meanwhile, Romo is a player with incredible instincts. He may not always know exactly where a defensive player will be or what he's going to do, but once the ball is snapped, his instincts take over and he can make decisions quickly.
I guess what I'm saying is that while Peyton excels in knowing what's going to happen before the snap, Romo does his thing after the snap.
This is not to say I don't think Romo puts in a lot of work before games, but it just seems to me that he plays much more in the moment, kind of like Favre, I guess. Which also means that while Romo does his thing well, there will be at least a couple of throws where everyone will go, "What was that?"
I'm also well aware that Peyton has years of proving himself, so I'm not saying Romo is Peyton by any stretch.
What do you guys think?