1) The Anecdotal Evidence: In 2008, Barack Obama was a challenger with no record, up against a non-incumbent. The Republican incumbent who was in office had an approval rating of 25% and a massive financial crash at the very end of his second term. Meanwhile, Obama had a 3-to-1 spending advantage, was drawing massive crowds, and was generating tremendous excitement while a lot of Republicans chose to stay home rather than vote for John McCain.
In 2010, the GOP had its best year in half a century and today, Barack Obama is an incumbent saddled with an economy that's limping along and an unpopular record. Moreover, because Mitt Romney didn't take federal matching funds, he has had the money to compete down the stretch while there's no sign at all that Barack Obama is generating the same sort of massive enthusiasm amongst young and black voters that he did four years ago. Yes, he'll win both groups, but his margin with young voters will be much lower and turnout as a percentage of the electorate is likely to be down for both groups.
This gets to the central question of the election: Is the electorate going to look more like 2008 or 2010? The anecdotal evidence suggests the numbers will be closer to the ones we saw in 2010.
2) Early Voting: In 2008, Barack Obama crushed John McCain in the early voting by a 55-40 margin. This was something his campaign was counting on doing again. Instead, both Pew and Gallup are finding that Mitt Romney is winning early voting by a 7 point margin. In state after state, like Ohio, Virginia, and Wisconsin, the evidence suggests that Obama's numbers are way down. This is very significant because Republicans tend to outperform Democrats on Election Day. So, without that edge in the lead up to November 6, Democrats usually lose.
3) The Flow of the Blow: At the end of the campaign, you're starting to see Romney campaign in states that were considered givens for Obama a few months ago. Michigan (16), Pennsylvania (20), and Wisconsin (10) are all in play and arguably, even Minnesota (10) and Oregon (7) aren't out of reach for Romney if he were to make some big ad buys. Obama is now in the same situation McCain was in back in 2008 when he was desperately playing defense in states like North Carolina and Indiana that are generally considered to be gimmie states for Republicans.
4) Independent Voters: Since Republicans and Democrats overwhelmingly vote for their own side, Independents are obviously very important. In 2008, Barack Obama had an 8 point edge over John McCain with Independent voters. This time around, polls show that Mitt Romney has a big edge with Independents. Although the numbers vary from poll-to-poll, almost all of them have Romney winning Independents by somewhere between 7-20 points. Just to give you an idea of how significant that is, the last candidate to win Independents by double digits was George H.W. Bush, who won Indies by 10 en route to a 426-111 electoral victory. Romney isn't capable of winning by that kind of margin, but if he takes Independents by 10 points or more, as a practical matter, it would be almost impossible for him to lose.
Because of the discrepancies in the polling data, no one can say with absolute confidence how this race is going to turn out. But, if you had to guess, you'd tell Obama's maids to make sure Obama's 35 million dollar mansion in Hawaii has all the mirrors shined up and ready to go before January 20, 2013.