Young talent on their rookie deals, like AgentDib put it, is huge.
I would add to the OPs idea that scheme fit is even more important. We saw how this defense looked switching to a 3-4 last year. A lot of the players on the roster didn't fit the new defensive scheme.
Adding to that, player chemistry. How will these guys play together on the field? The LOB had such amazing chemistry, they knew where the other D back was going to be, and they played like a well oiled machine. Also, if they get along in the locker room/off the field, it will make it much easier for them to communicate amongst themselves and be a more cohesive unit in general.
Coaching is definitely important as well. I think our issue last year on defense was the coaches not putting players in a position to succeed. When players are consistently looking around after a big chunk play with a confused expression on their faces, that tells me that the coaches aren't effectively communicating to the players, causing the players to not communicate amongst themselves.