Yes, the run defense must get better, but Barkley's going to make quite a few run defenses look bad this season.
And yes, the DL and LBs must play better.
But my overall concern is the defensive philosophy on 1st and 2nd down, with formations that are easy to read, no imagination, and few run blitzes. They need to threaten the LOS more with 8 and 9 in the box, and they can just drop at the snap to protect against a big play. They need to get into the opposing QB's head a bit.
Conversely, that's why Moore's offense seemed to effective. All the pre-snap shifts and adjustments keep the defense guessing, and makes it harder to predict what's coming at them from snap to snap. Basically, the Cowboys need some Moore-like imagination on defense, so they're not always just sitting ducks on each play. More pre-snap movement, and more deception. On defense, the Cowboys are playing Checkers, and just hoping that the Cowboys players win all the individual matchups. Many times, the offense is playing chess.
Yes, the Dee may give up the occasional Big Play, but the soft Bend But Don't Break philosophy is just not a recipe for intimidation or long-term success. Yes, BBDB limits these Big Plays, but it's so passive and easy to exploit. Ultimately, it saps the enthusiasm out of the front 7 on the early downs, and it negates the abilities of our fast, young LBs, who can quickly shoot gaps and run down RBs and QBs on the backside. I feel like the Cowboys are always waiting to simply get to 3rd down to blow things up, but there should be more attempts to wreak havoc in the early downs.
Right now, the Cowboys O looks somewhat intimidating....they need a similar threat on defense. It starts with coaching and the Coordinator.