Youth and inexperience, primarily. Lack of clear leaders at some positions.
But that's boring. And too broad. And too short for a blog post.
So here's the more detailed deal, as I far as I can discern it.
The good news: In the defensive system the Patriots run, stout linemen are the linchpin. And this year, the linemen, specifically veteran nose tackle Vince Wilfork, have become for the young Patriots defense what Brady has been for the offense -- an anchor.
The bizarre case of Richard Seymour still leaves some question marks, but even after Wilfork's fellow veteran Ty Warren went down, Mike Wright and a cast of thousands have filled in from game to game, and it has worked. More than worked -- at this point, the D-line is at the heart of the Patriots' developing defense. Wright had 1.5 of the Patriots 4 sacks on Ben Roethlisberger last Sunday. Veteran outside linebacker Tully Banta-Cain had another 1.5, but it's the hogs up front that let the linebackers do their work. (Also, once again, w/r/t Seymour, Belichick has come out on top in the long run, after a player's contract dispute.)
After what happened with Seymour, I was prepared for anything when it came to Wilfork's contract this past offseason. But of all the seemingly indispensible Patriots defensive veterans Bill Belichick has come to the negotiating table with over the years, Wilfork is the one who's gotten what he wanted. That ought to tell us something.
The bad news: At some point the revolving door going on with at least one of the defensive end positions should slow down just a bit. Some people like Ron Brace. I'm partial to Myron Pryor. We'll see who sticks, if anybody.
Good news: Devin McCourty continues his ascension, in the court of popular opinion as well as in the game stats. From Week 1 when the Bengals tried to pick on the rookie, McCourty has pleasantly surprised. He led the Pats in tackles and assists Sunday.
Bad news: Kyle Arrington has been surprisingly good at times, but mostly a mixed bag. Jonathan Wilhite has been 'eh'. Darius Butler was awful, and that's all I have to say about him. .
Good news: Inside linebackers. Gary Guyton had a sack Sunday, and Jerod Mayo will hit the taste right out of your mouth (I still don't think that the Steelers got past him on one of their two-point conversions in the fourth quarter, even though it was ruled that they had.)
Bad news: They had a great game against the Steelers, but as a unit these players have taken their time finding a rhythm this year. The Patriots are still tinkering with the depth chart at outside linebacker. And they may be coming together, but with the potential exceptions of Banta-Cain and Mayo, they haven't come close to replacing lost leaders like Mike Vrabel and Tedy Bruschi.
Good news: I was actually kind of surprised that James Sanders won AFC Defensive Player of the Week, when it is Patrick Chung's return I will remember most about this game, defensively speaking, and the sound of his hits through the television -- like the sound of a baseball off the bat of David Ortiz, there are some players, like Chung's predecessor Rodney Harrison, who just have a trademark, audible oomph when they make contact. Like Harrison, Chung seems to be emerging as a leader for the young secondary, and was second among Patriots defenders in tackles and assists Sunday even though it was clear his knee is still bothering him. He even tipped the ball that Sanders grabbed and ran back for the pick-6, which is pretty much what earned Sanders the Player of the Week.
But I believe this is called one of those good problems to have.
Bad news: Brandon Meriweather. He was on thin ice from a mental standpoint coming in, as far as I was concerned, and his hit on Todd Heap still sticks with me, especially as he continues to draw penalties and make other mental mistakes.