Sean Trende believes that millions of white voters failed to show up:
[I]f our underlying assumption — that there are 7 million votes outstanding — is correct, then the African-American vote only increased by about 300,000 votes, or 0.2 percent, from 2008 to 2012. The Latino vote increased by a healthier 1.7 million votes, while the “other” category increased by about 470,000 votes. This is nothing to sneeze at, but in terms of the effect on the electorate, it is dwarfed by the decline in the number of whites. Again, if our assumption about the total number of votes cast is correct, almost 7 million fewer whites voted in 2012 than in 2008. This isn’t readily explainable by demographic shifts.
In the end, total turnout will probably approach '08 levels, although it's unclear whether it will ultimately exceed it. Romney will probably end with more votes than any candidate in history who isn't named Barack Obama, demonstrating a strong GOP turnout. Given the initial exit poll data and the current national popular vote tally, it looks like white turnout might have declined slightly since 2008. I'm holding off judging just how much until the final exit polls and results are reported, but I'd be quite surprised if it turned out to be 7 percent–especially in the battleground states.