“Anybody can play. The note is only 20 percent. The attitude of the motherfu&#r who plays it is 80 percent.”
― Miles Davis
“It’s not the notes you play, it’s the notes you don’t play”.
-- Miles Davis
“I always listen to what I can leave out.”
― Miles Davis
Re the basic question “What makes a good musician?”:
Some good comments so far and I agree with the OP’s basic premise (and Miles’ 😎). Of course, having great chops does not always mean possessing great musicality. Having said that, having great technical skill sometimes gets a bad rap. Great technical skill opens musical doors that remain shut to players with limited technical skill; even when they are fundamentally good musicians. The important point is that one doesn’t necessarily have to, nor is it always musically appropriate, to go through those doors. A player possessing limited technique can still be a good musician, but it can be.....limiting. While those flurries of notes and/or “complexity” that some players hope will add meaning to their playing are ultimately meaningless and even destructive, there are times when they are just the ticket and what serves the music best. If you don’t have the chops you can’t go there.
(Btw, pet peeve. and not meaning to take this in a different direction. “Musicality”. IT IS NOT AN AUDIO TERM! Musicians, not gear, possess musicality).