A lot of easy ones when it comes to player dissappointments... but I'll go with a positive.
The emergence of Matt Belisle. I personally had very low expectations. I was pumped about getting him originally in the Mercker deal with ATL, but I slowly grew less excited, thinking of him at best a decent middle relief guy. He's really shown a lot to me and I wouldn't be surprised if he gets the 5th spot in the rotation sometime this year.