Jacksonville Jaguars, it was the offensive line. Porous against most pass rushes and average at best in the run game, the offensive line struggled more than any other group on the team.
It wasn't just the quality of the players, but various things like changing quarterbacks and injuries never allowed the offensive line to really grow as a unit. It's an unappreciated aspect of a positional group, and it was apparent last season.
Letting up 71 sacks in a single season is the most popular stat thrown when it comes to the offensive line, but it's not the full story.