Sticking to the pitching side of the house, biggest disappointments for me are Corcino and Guillon.
Pretty much all the others are excelling. Cingrani, Stephenson, Rogers, Contreras, Cisco, Moscot, Romano, Crabbe are looking good. Reynolds may have pitched himself into a major league job somewhere. Some team may come asking for him if they have a hole to fill. Even Galaragga (spelled with 4 A's for a reason IMO) is doing well. Others, Muhammed as noted, Carlos Gonzalez, Loek Van Mil and Brian Pearl aren't doing bad so far.
On offense, not sure its a surprise, but given the Reds need for a RH OF bat and the lack of candidates in AAA, it may be time for Bryson Smith to get a promotion to AAA to see how it goes.