Peyton Manning was a guest on Jimmy Fallon when the topic of Eli Manning's Super Bowl face came up. Peyton tried to explain it before the two of them made fun of it.
Of course this isn't Eli's first Manningface and will not be his last. In fact he had a similar look on his face two years ago when he watched Peyton lose.
It must just be his game face. Some people have a mean look Eli has the "my puppy just died" look.