For any multiplayer game, what makes a good player is the Three Aspects:

1. What you Know.
2. What you Have.
3. Who you Know.

What you Know is what everyone commonly associates with 'player skill'. This is your knowledge of the game, the mechanics of an encounter, class abilities, synergies between classes, and you're ability to both learn new things and knowing which resources to use to advance your knowledge of said new content. If you've ever seen the 'new' player excel at the content like a trial or raid, its likely they are strong in this aspect. They've been able to learn the content, research it, and/or adapt to it.

What you Have is quite simply your character, their equipment, and levels. It also represents your wealth as a potential to increase your character's influence in the encounter (tanking, healing, dps, ect). Being well geared simply increases the potential of the other two aspects. That makes it important but not in a complex way. Being ilevel 340 simply means the same person will be much more effective than when they had 300.

Who you Know is a interesting aspect and one that is commonly ignored or overlooked. Who you know is on one side your personal network. That being your friends, free company, statics, faction (if in PVP), your world, and your datacenter. The first part of this aspect is simply who you have at your disposal when clearing content. The better the pool of players the more successful you'll likely be. The second part is your personal charisma, leadership, teamwork, and general gregarious ability. Its how well people like you, how well people interact with you, and how well you work with others.

These three aspects synergize with each other in a very organic and dynamic way. For example, having good gear means little if you don't know how to play the class to its fullest potential. And surrounding yourself with people, or generally having a bad attitude means clearing content will be arduous at best. You must at all times strive to increase the potential of all three aspects and every part of them. This means to be a good player you must learn the game to the best of your ability, gear yourself appropriately, and generally be a good person and work well with others. Lacking any of those aspects will put you at a disadvantage compared to someone who doesn't.

This affects ALL multiplayer games without question. The balance of the three aspects is also more pronounced in competitive environments where players are measured against one another. This could be competing for spots in statics, raid groups, or in PVP scenarios. If you ever find yourself struggling in content in any game, match yourself to those aspects to see where you are lacking. Remember to ALWAYS be honest with yourself. Sometimes you lack the skill or knowledge when others don't. Sometimes swapping out a piece of equipment is all that is needed.