I am not concerned with the difficulty of content that a player can clear when I consider attributes that a good player displays. Players who make the effort to work as a team are good players. There is more involved with teamwork than just skill.

Skill is important and players who don't make the attempt to know their jobs are difficult to have in a party. I think it shows a lack of respect to not make the effort to understand how your skills work. In the same way, players who don't know what their role should be in a party or who refuse to do their role in a party are not good players. Dps players who consistently run ahead of the tank and pull mobs come to mind. That said, I am very patient with players who are learning their job or their role in a party. I'll happily run the same content wipe after wipe with players who are making the effort to learn. You never know; the player who is struggling but getting better in your party might turn out to be the next great raid leader after some practice. The important part here is that a good player is making the effort to improve because they want to contribute their fair share to the party.

Another important element to being a good player is being able to play well with others. Some people treat group content like single player content and that mentality makes for frustrating and difficult runs for the whole group. Some other very skilled players expect all random duty finder players to be as skilled as they are and get rude when party members are still learning their jobs or are familiarizing themselves with some new content. I don't like to be in parties with either of these two types of players. I love being in parties with players who treat the other party members as people and not as NPCs and with players who allow for the less experienced to learn.

There are good and bad players at all skill levels. To me, what matters more is teamwork and a willingness to grow as a player. A friendly demeanor goes a long way also.

Good players are excellent to each other.