2013's most improved rosters

Star Lotulelei helped the Panthers climb back to the playoffs in 2013. Grant Halverson/Getty Images

2013's most improved rosters | Biggest drop-offs

Parity is a huge facet of the NFL. The salary cap and small talent margin between teams ensure that it takes only one injury, one strong draft or one excellent free-agent pickup to push a team that was languishing in the division basement to the very top.

So who saw those types of key improvements in 2013? We're turning to our NFL player grades to see which five teams made the biggest leap from 2012 to 2013. We focused on which existing roster players stepped up big, which newcomers provided the most help and which units raised their play to fuel their team's surge.

Topping our list is a team whose coach traded the hot seat for a new nickname and a spot as the NFC's No. 2 seed in the playoffs.

1. Carolina Panthers, +240.3 increase

Stepping Up: We'll get to why they were so much more effective, but there's no denying that the duo of Luke Kuechly and Thomas Davis delivered in 2013 with their combined grade shooting up by 12.0 points. Most of the hype has been directed at Kuechly, the tackling machine who was cast as the figurehead of the Panthers' resurgence. But it's actually Davis who graded better, chiefly because of his work in coverage. In an increasingly pass-happy league teams need their linebackers to be able to cover, and Davis is really something special in that regard. He earned the highest grade of all 4-3 outside linebackers in that facet of play.

Big Addition: Rookie GM Dave Gettleman didn't need long to figure out the team was weak at defensive tackle. It wasn't just about the team needing to get some space eaters, but the Panthers needed penetration in the run game and against the pass. So they drafted Star Lotulelei and Kawann Short. The results saw their defensive tackles grade swing from minus-45.2 in 2012 to plus-23.8 a year later. Lotulelei was a huge part of that with his work in the run game, finishing with a 12.9 run stop percentage that was second best of all defensive tackles.