I'd be willing to expect there'll be as many answers to that question as there are for what defines a "good player", which is to say "a lot". People will have many different perceptions and opinions on the matter. I've said before that I believe a "good" player is one who has a good attitude, communicates with the party when needed, always considers their party members needs and does their best to react and adjust as needed, and has a desire to try to improve themselves when they can.

With that in mind, a "bad" player to me would be one who eschews these virtues, one who is selfish and has no regard for anything other than doing what they want even if it's counter-intuitive to the party's progress, and has no interest in understanding or developing their skills, expecting others to essentially carry them through any content they want to clear. Of course, such things don't apply to all content within the game, but much of the game has a focus on working with others as a team to achieve a goal, whether it be something a simple as a standard dungeon or as tricky as Savage raids. Even things as casual as certain seasonal events still have instances where you need to work with others. Even outside of team content, it's important to show respect for other players you may run into on your server as well. You never know when you may end up joining them in a party, after all.

Now of course, no one's perfect. People make mistakes, whether it's in content or even a social misstep. What sets someone apart is what they do afterwards. A good player will acknowledge their error, apologize, and will try to learn from the experience. A bad player will, of course, do just the opposite.

In short, I don't judge a player by skill alone. After all, skills can be improved upon, and we all start somewhere and also progress at different rates. Patience can be an important virtue as well, particularly where at least one person is still learning.