I believe that scheme and personnel is the main culprit to the Pats problems.
Although it seems this way because we're so familiar with them, but I find their offensive scheme extremely predictable and believe O'Brien has a terrible feel for the game. For example, games against the Falcons, Ravens and Colts should've been blowouts if it weren't for poor game management. The problem I have with the offense is that they throw when they should run and run when they should throw. It's seems as though the Pats are trying way too hard to out trick the opposition. Before the Saints game, the Pats would continually try to throw on 3rd and short when they had capable backs such as Taylor and Morris. In fact, it leaves you thinking, why the hell did they even sign Taylor in the first place? In addition, most of us can agree that it's extremely frustrating to see the Pats running game get into a rhythm, yet week in and week out, we see the Pats going right back into shotgun and passing way too much. Last Monday night is a perfect example. Just when Maroney was getting into a rhythm, the Pats say "thank you very much Laurence, now you can go hang out on the sidelines and watch us pass the ball." Not only did they take a chuck of clock off of their first drive, the Pats were in perfect position of dictating the tempo of that game. This has been the theme the entire season.
As far as personnel goes, I don't know what Belichick sees, but they don't have the talent up front to throw like crazy. I used to be a Kaczur defender, but I can't do that anymore. Since 2008, he's been terrible. Like Kaczur, Koppen has struggled and continues to get blown up in the backfield whether it's run or pass. Also, Neal gets injured way too much and they need to find a young replacement in the off-season.