I believe turnout could have been lower, but the lower turnout should have come from Obama's base who were less enthused about the historic nature of the election, and perhaps otherwise disinterested voters who voted in 08 to be a part of history.
At the same time, I expected Republican turnout to be higher than 08, given higher enthusiasm for Romney than McCain.
I think it is state-by-state turnout that needs to be looked at, because the national turnout total is nothing by itself, it is the sum of the turn out in the states, and there may be some states that hold a major piece of that lower turnout all on their own (like storm ravaged New York and New Jersey - both so “blue” even very low turnout would still be an Obama win).