For me the biggest disappointment was just simply the sheer number of injuries.
After that I would say the lack of cohesion along the OL and blocking skill position players. There weren't a lot of "clean" plays where reads happened in a normal flow.
Third would be the lack of development along the DL. The lack of sacks, pressures, or negative plays of any kind (without bringing numbers) made the rest of the defense look very average. When you watch the rest of the CFB landscape play (over bowl season) you realize that most defensive stops end up coming on the heels of an offensive penalty, or somebody stepping up and making a play putting the other team behind the sticks. One really good play combined with a bunch of average other plays usually gets it done. Most stops don't come after 3 or 4 positive gains in a row. Also, the advantage of having 7 guys in coverage while still getting pressure with 4 is undeniable. For GT to get pressure, we needed to have only 5 or 6 guys in coverage, big difference. Gotsis is the exception and his loss, whether to injury or penalty was crippling.
Fourth, would be skill guys simply not getting any extra yards than were there directly as a result of open space. Said another way, we didn't make 7 yards out of a 4 yard play very often at all. That wasn't the case last year.