Egg-shaped pumice, chipped bone rod.

[Sarkov]

You probably notice the difference between a good player and an average one most for Bard and Cleric, but all classes have some pretty substantial skill checks. General stuff that applies to being a good pvper of any class (buff slots/clickies, dispels/cures, resist gear, crimson pots, situational awareness, ability to follow/assist, etc), and a lot of class specific stuff.

Pet control in particular stands out as an area where average players are outclassed by good ones, both keeping your own pet free of cc and controlling other players' pets.

Jousting (really just minimizing the amount you get hit vs. the amount you hit them) is another obvious one applicable to any melee, as is disc usage once Kunark rolls around.