Depends on the context of the situation.

A good new player, to me, is someone who is open to suggestions/help with how to improve themselves.
A good veteran player is someone who is able to play their class effectively(not perfectly, but not hindering everyone), and not ignoring all mechanics because stuff is "faceroll" easy.
A good mentor is someone who can give advice when they know what they are talking about, admit when they aren't as familiar with something as they probably could be, and gives it their all when getting something they don't like via mentor roulette.