A good player is an asset that pushes the team to victory through whatever is needed at any given moment, AKA playing objectives. Sometimes the objective is to recover a flag, assist a teamfight to free up players so they can move on to different objectives (this is very different than the usual "fighting at mid" thing), etc. The skillset necessary for someone to be a good player, from this perspective, is relatively simple but uncommon, basic class knowledge, of both your class and your enemies', and situtional awareness. The latter being the most important as it allows you to take informed decisions that dictate your next action towards team victory.
A bad player is anyone, regardless of skill level, that does not contribute towards the team victory. It does not matter how many BG/arena addons, macros and whatnot this person has, it does not matter how good he is at his role (dps/heal/tank) and inflating numbers, nor does it matter how good he is at 1v1. This is not an argument against the lone wolf kind of player (as I am one myself), but plain and simple saying that if you do not play objectives, then you are not a good player.
TL;DR
Instanced PvP is a team effort, if you use your skills to push your team to victory, then you are a good player. Anything else is just bad.