One of the main points to come up on my last visit to see my sensei was 'grip'... It was interesting to see how highly he ranked it in importance, however following the logic it was easy to see why...
Consider the effect of gripping the tsuka too tightly... the arms tense right through into the shoulders and they are pulled forward. Posture gone, balance is out, and you cannot swing the sword in an natural manner. In extreme cases, the shoulder blades can 'wing out' away from the back and cause serious physical problems, although this is more related to over-reaching..
Gripping with the front fingers, not the little finger only: You get what he called a 'baseball bat' grip, which shortens the 'diameter of the swing' considerably, again lifts the shoulders, and falsely stops the sword early rather than allowing it to naturally cut through to a natural finishing point.
Position of the grip: If the hand is off to one side, not above the tsuka, you are trying to use the thumb as part of the 'grip'... as the thumb has a low muscle strength, when the cut hits the sword will fall out of the hand/bounce, depending on how bad the position is... (At this point it is worth pointing out he does not teach shibori for MJER, as the grip must be automatically correct as you get hold)
Tightness of 'grip': There is an interesting experiment you can try... watch what happens to your shoulders as you increase 'grip pressure' and add in the fingers you should not be using...
The lecture/demo went on for over an hour, with more useful hints and tips, some I knew, others were new to me...I have seen my grip change over the years, and this reinforced what I need to pass on to my students...and how to correct them.