Who is this guy?

I couldn't help thinking the following as Barack Obama made his "surprise" appearance at the end of Joe Biden's speech: Who is this young, thin guy coming out to emcee the festivities? Now being young and thin shouldn't be held against someone, but I mean, really, what is this guy's track record that qualifies him to put us at his command? His lead? He was only just giving a keynote address at the last convention four years ago. He was completely unknown then, and he's still really new to a lot of the country now. What has he done at the national level or at the state level that is known nationally? Obama's got to win the election, but after reading about and seeing him, I also feel that Obama can sometimes be getting a little too big for his britches.