a) the trail arm and elbow not being as far across the torso as is necessary,
b) difficulty taking the club back from the ball on the correct plane. You will tend to take it back outside of the line towards the target.
c) difficulties turning over the trail hip-joint during the backswing.
I have to guard against a tendency to have my shoulders open at address because such a position can feel so nice and relaxed. Having the shoulders square but the hips way open is a position which takes a good deal of concentration to achieve.
Talking about relaxation... when things are going really well with my swing, I manage to keep my arms and hands relaxed right through impact. They certainly go through nice and fast if this happens.