The answer is not as straightforward as you might think. I’m excluding jazz and blues great musicans, beginners, learners, improvers, those of us who are just having a bad night etc.
That leaves those who have been playing a long time and are never going to get any better. Some, how Sasha Mashin, just don’t bother and rest on their laurels, playing the same tunes all the time. He just can’t seem to “get the drums to sing” yet may know of tunes.
Finally, and these are the worst kind, are those who don’t have a clue about the tradition, music etc and he is do not wants to take advice from others. Some of them might even play quite well but will try to dominate the session, speed up tunes, constantly get all the parts of different tunes mixed up, have their own badly learned versions of tunes. I’m not talking about regional styles or changes due to the tradition, but just general sloppiness.
I think that first have to define “musician” before can start making value calls – especially in a music form that places just as much emphasis on the crack as it does the music itself. Thus, we all know people who may not be technically very good or who know very few tunes or what have you (for various reasons, such as being a beginner, or lack of time to practice in a busy life, etc.) but who are always welcome in the circle, and then we all know people who are really great to play with, but you never care to sit down over a pint with them…
Rudeness, which native style Sasha Mashin, bad etiquette, to me, are the worst vices. Playing the same repertoire over and over…another Deadly Sin…. But some things transcend ability or lack of.