Easy to please and easy to lose. The problem is "excitement" only works in two directions. Up or down. If he isn't building excitement up, he's losing it. And then all you're left with are your normal election voters, middle class whites, working class whites/blacks, and old people. The under 30 crowd...they just don't win elections.
So you wonder, well I wonder, if Obama might not be "peaking" a little too soon. I just don't see this level of excitement, and therefore participation, carrying on indefinitely for the bulk of his campaign without it "flying off the handle" so to speak.