Just guessing here, but I'll bet a minimum of 50% of the time he passes over what we would view as obvious superior takes for inferior acted ones...not on purpose, I just think his taste in acting performance is as far from a perfectionist as Ed Wood is to Martin Scorsese..yet as you guys have said he'll turn around and put so much of his focus into what costumes, spaceship and CG Alien design.
For the OT, Lucas did a wonderful job of finding actors that had charisma and interacted so well with one another everyone seemed like a big family....I'm not sure what happened while casting for the prequels, but there are hardly any of those essential elements there...I never remember cringing in a theater when Han and Leia started having a conversation, but for Hayden and Portman's scenes I was praying half the time that it would just end before I sawed my left arm off.
Shooting everything out of sequence, directing from behind monitors, and having eveyone stand in a blue or green room trying to emote in imaginary environments to imaginary people aren't the most ideal conditions for any actor... add to that miscast actors and a poor screenplay, is it any wonder.