A new director would have been the difference between night and day. It was originally GL's plan to have someone else direct each movie, then he decided, being the kick-ass greatest director of all time that he is, to do them all himself. Sigh. Most unfortunate.
The PT was in serious need of someone, anyone, taking a few steps back, looking at the larger picture and saying, "I've got a bad feeling about this..." and making adjustments so as to invoke good feelings. Unfortunately, George was unable to do this, and obviously none of his creative team had the balls to tell him that the masterpiece he was painting looked like a kid's drawing scribbled in crayon.
An outside director was exactly what these films needed. Too much control was in the hands of a single mind, and single set of eyes.