i hate doing these type of things, because I try to have a positive outlook (well at least the old HammondsNats tries to), I'll give it a shot:
Breakout - Ian Desmond
I truly think this kid is going to have a good year, if he's given the chance. He brings energy to the ballpark everyday and not only will he put the Nats on highlight shows (with his glove and from time-to-time his bat), he will help them win some games. If he is given the opportunity and can deliver on it, I think Espinosa may have to move to 2B (and compete with Kobernus).
Disappointment - Jesus Flores
Have to agree with epic_phalanx ... I think Flores has all-star talent, but the injury thing is getting really old and who knows if he can shake it. If Norris can step up his defensive game and handle A/AA pitching and if Flores can't stay healthy, we may have our new catcher of the future. Thank goodness we signed Pudge.