It's difficult to say. Senatorial elections that take place during presidential elections depend in large part upon the coattail effect. So if the Democratic presidential candidate does extremely well in 2016, Democrats will pick up much more seats. One of the reasons why Republicans are expected to do so well this year is because six years ago, this is exactly what happened when President Obama ran. Democrats rode Obama's coattails to victory, but now they have to defend red states like Montana, West Virginia and North Carolina without the benefits of a popular president or a presidential election.
Let us assume that Republicans in 2014 win the states they're currently polling better in, resulting in a respectable seven pick-ups and 52 seats in the Senate. Democrats would only need to win three seats to take the Senate back, or only two if a Democrat wins the presidential election.
The Senators elected in 2010 were overwhelming Republicans (this was the Tea Party election, you may recall). Here's a list of every Republican-held state that will be defended in 2016, along with their chances of winning, absent a major political upheaval favoring one party:
States that it is almost certain Republicans will win (huge margin of victory in 2010, or a veteran Senator): Alabama, Arizona, Arkansas, Georgia, Idaho, Iowa, Kansas, Louisiana, North Carolina, North Dakota, Oklahoma, South Carolina, South Dakota, Utah
- Note that I place Iowa in this category on the assumption that Senator Grassley decides to run again. If it were a new Republican candidate, the race would probably be a toss-up.
States that it is likely Republicans will win (good margin of victory in 2010, or a well-liked Senator): Alaska, Florida, Indiana, Kentucky, Missouri, New Hampshire, Ohio
- Florida, Kentucky, New Hampshire and Ohio are on this list solely on the popularity and political clout of their current Senators (Marco Rubio, Rand Paul, Kelly Ayotte, and Rob Portman). The problem, of course, is that all four have been mentioned as potential presidential or vice presidential nominees. If any should decide to make a run, it could likely throw their state into toss-up territory.
States I think would probably be toss-ups, if not a straight-up Democratic advantage (slim margin of victory in 2010): Illinois, Pennsylvania, Wisconsin
- Illinois Senator Mark Kirk has announced that he is running for reelection, otherwise I'd write that off as an obvious Democratic pick-up. But even with Kirk in the race, Illinois will probably change hands; he only managed to win in 2010 because of the Republican advantage.
According to insiders, the five states most likely to be targeted by Democrats in 2016 are Illinois, Pennsylvania, Wisconsin, New Hampshire, and Ohio. By contrast, the only two Democratic seats I could see Republicans being competitive for are Colorado, based on its purplishness, and Nevada, based on how remarkably disliked Harry Reid is in his own state.
tl:dr This all depends on a million factors, but I think it's well within the realm of possibility for Democrats to take the Senate. Especially if Hilary Clinton runs for President while maintaining her current popularity. But of course, I would have told you that Republicans were a shoo-in to take Missouri in 2012 and that obviously Scott Brown wouldn't win the 2010 Massachusetts special election. Politics has a way of throwing curve balls.