I think two things leap immediately to mind
1) The DBs actually battle for the ball and get physical in coverage. This makes a world of difference, because last year, even when they pressed (rare, I know), they would mostly just try to stay around the player they're covering, hoping the QB wouldn't throw their way. When the QB did, they didn't know what do, because they were too afraid of drawing PI flags. This year they seem to know how far they can push it without getting flagged.
2) The tackling. My god, the tackling this year is so much better. Those cheap little screen plays went for 30+ yards with regularity last year, often 80, and it boiled down to an inability to tackle the ball carrier in space. Penn State ran, what, 15 screens? Some of them might have gone for ten yards, but none of them went for much more than that, because DBs and linebackers were there to make tackles they never could have made in 2013.