Leonard Williams is set for his breakout year. He was already one of the best run defenders in the league during his rookie campaign, but he had to work on his pass rush moves.
The Jets have had the best run defense in 2015 and it wasn't even close. Damon Harrison was a big loss but with McLendon and Richardson to bounce back I expect them to hold the same level.
Their problem was a lack of speed and edge rush. They weren't able to turn more plays into sacks & hits as they should have been. Laurenzo Mauldin was a good situational pass rusher during his rookie season - he should make a step forward and see more playing time. Darron "Speedy Gonzalez" Lee will make an immediate impact on all three downs.
It looked like Darrelle Revis declined at bit at the age of 30 and while he looked indeed sloppy in man coverage at times, he allowed the fewest completion percentage of all CBs at 44.2%. Another problem has been his wrist injury. He got surgery early in spring. Revis can still be an island and takes away a lot of the field, especially in zone coverage. Here's an example: https://www.youtube.com/watch?v=k6o-1W1McyU#t=2m20s
Letting Antonio Cromartie go was a very good move as he was just terrible. Him not finding a new home is saying it all. It doesn't make any sense to let Buster Skrine play on the outside. He is a slot corner and doesn't suit well as an outside corner. I think Marcus Williams and rookie Juston Burris will compete for the outside corner spot next to Revis. With Gilchrist and Pryor, the Jets have a good safety tandem and the secondary as a whole should be improved from last year.
I will be very careful with the Jets this season as they have a terrible schedule, but one thing is for sure: this defense is loaded.