But you would not tell them to their face, for fear of embarrassing them or hurting their feelings.

Anything from how they move or angle their hips or how they thrust or don't thrust or something they can or cannot do with their body, as you would desire.

Examples - If she cannot arch her back up in doggy position, to push her pussy out and back. Some women just cannot do it because of anatomical reasons or because they do not know how. It is no big deal, but also is for some men.

Or

If he does not know how to thrust up into her when she is on top. It is a tricky alignment and balance to hold steady and can throw the rhythm off for the woman. Again, not a big deal, but also is for some women.