Republicans have a steep, steep advantage because of the playing field itself. Remember, this is the rematch of the 2008 election, another year like 2006 when basically every Democrat who had a chance won. That means there are a lot of freshman senators who are only barely plausible in their states facing reelection. Add to that that it's the mid-term in a six year of a presidency - which favors the out-party and the GOP, which in this case happen to be one in the same. Some have, despite expectations, nicely firmed up their positions in their states to the extent that they face only token opposition. Al Franken is probably the best example of that. But many face challenging battles.
The real question is whether the Democrats can rely on the improbable ally of the Tea Party which has saved their hides in the last two cycles. And it may just happen. Kentucky, Georgia, Colorado and a few other states show a real chance of replaying the 2010/2012 model where Republicans blow pretty solid odds by nominating a candidate who's simply too nuts or offensive even for a purple or fairly conservative electorate. Mississippi has a truly nutball Tea Partier gaining traction. He's close to be an out neo-Confederate and crypto-white rights candidate. But the state is so conservative I doubt a Democrat could beat him regardless. So that's likely just an opportunity to have a semi-closeted white supremacist in the Senate.
But will it be enough. Here's Charlie Cook's current rating of the race. The one race I think he's off on is Michigan: that's not a toss-up. Is there an Angle or Akin or O'Donnell? Georgia's the best shot at that. And Dems are lucky to have a pretty strong candidate in that race, despite having a fairly weak bench in the state. But are there enough? That's the question.