Well, we've got a structural edge right off the bat in this election. Despite the fact that we hold more Senate seats than the Dems, more of them are up for re-election this time around than us. They've also got quite a few retiring (I count Jeffords as a Dem). More exposure = fewer chances for them to gain seats, more chances for them to lose seats; fewer chaces for us to lose seats, more chances for us to gain seats.
Remember Jefferts left he Republican party and went Independent/liberal