Manning looked very nervous indeed before the game started... and the "bitter Super Bowl face" came out in full after the disastrous first play. The game was for all intents and purposes over at that point. The Broncos needed to rally behind their leader, but their leader was lost in a daze. The rest of the team responded accordingly, and the Seahawks fed on that.
Manning is probably the greatest QB ever when it comes to preparation for a game. He is very robotic in nature though, and in times when things don't go according to plan, he isn't very spontaneous at all. Much of that is mental, but some of that is due to his physical limitations (ie. lack of mobility & arm strength), particularly at this late stage of his career. Kaep being a physical freak of nature is better equipped IMO to make things happen in those tough situations where Manning often fails. It's early, but he also appears to have an advantage in leadership traits. Barring serious injury, I can see him getting multiple rings before his career is over, but only time will tell.