I think the best addition for the run defense is Cushing. The biggest weakness in the run defense was Diles who got washed out on alot of plays on the strong side. He wasn't strong enough for the point of attack and is a much better fit on the weakside. Cushing is a much stronger run defender, take on blocks and blow up offensive linemen, tight ends and fullbacks.
An excerpt from his draft card:
Can be overaggressive at times, which leads to some false steps and poor pursuit angles. However, he flies around the field and is a sideline-to-sideline run supporter. His motor is always revving. He is strong and shows the ability to hold ground when teams run at him. Uses strong hands and powerful upper body (30 reps on the bench) to stack the edge and rip off of blocks. He is a punishing striker with explosive short-area power.
http://insider.espn.go.com/nfldraft/...4453&year=2009