The biggest hole is the outfield. Maxwell and the 2 Martinez's are 3 black holes IMO. They signed Rick Ankiel...they could hope he turns it around I guess. Springer and Santana are their best OF prospects but neither are ready for the bigs (especially Santana). So if they were to go after any FA's, the OF is the spot that needs to be addressed the most.
On that front, I'm kind of surprised that they didn't make a play for Delmon Young. Than again, they may have figured that Young would be a negative influence on the youngsters, which is certainly understandable.