Dax Hill is an obvious pick for a list like this, and the way the coaching staff has handled Hill has been perplexing, to say the least.
In the 18 games Hill played this season (including the playoffs), he played 10% or less of the defensive snaps in 13 of them. I understand that he has Bell and Bates starting ahead of him, but it has been widely accepted that they drafted Hill in the first round to replace one of them, and if you plan on having him fill a Bates or Bell-sized hole in the defense, you figure they would've given him more playing time this season so he could get accustomed.
Still, at times, Dax wasn't really doing himself any favors. Just take the Wild Card game vs. the Ravens, Hill had three penalties, all coming from his snaps on special teams. He's a rookie, he's going to make rookie mistakes, and I think it's way too early to worry about him being a bust, but that's still a very ugly stat, especially considering he only had more than three tackles in one game this season.
Again, he's a rookie, mistakes are expected-- even CTB had some ugly moments this season-- and I'm positive he'll improve next season. But this was still a very underwhelming rookie year.