It is hard to isolate the characteristics shared by good game masters. Ask 100 players what makes some GMs better than others, you'll probably get a 100 different responses. Still, in my experience gaming over the years, I've noticed there are common qualities among the best referees.
I've also observed that no GM is static. Someone who runs great games 52 nights of the year for three years, can fall out of this pattern and go from a great Game Master to a mediocre or terrible Game Master. Like anything else in life, bad habits can emerge even the most practiced. By the same token, terrible GMs can grow and become magnificent.
That is why lists like this are sometimes handy. Even those who are demons behind the screen, occasionally need to go back to basics, and everyone can improve, no matter how good they are. These are just the habits, personality traits and behaviors I have observed in good GMs over the years.
They are prepared: This one is a bit of a no-brainer, but it is the easiest thing to neglect. Good GMs show up ready to run the game. What preparation that entails will vary from one person to the next, but you can always tell when someone shows up without having done enough prep. Preparation isn't just game notes, it is having a handle on the system, being organized with your gaming material, and being ready to play when play begins.
They are relaxed: In boxing they say the victor is often the most relaxed fighter in the ring. This holds true for many things in life. When you are relaxed, you perform better. When you are nervous or tense, you are more prone to flub. A little stress is good. Someone who is too relaxed won't even bother to keep the game exciting. Still, I see a big difference in performance when the game master is at ease. I find this in myself as well. I've noticed when I first game with a group of new people, things feel a bit off for the first ten to fifteen minutes or so until I get comfortable and start relaxing into the role of GM. So relax, relax, and enjoy the game.
They are not worried: This pairs with the previous trait. When game masters worry too much about outcomes, focusing on the destination rather than the moment, it affects the player's enjoyment. By outcome I simply mean whatever expectations the GM has in mind. This isn't about railroading so much (though it includes that) as it is about fretting that the game go perfectly, that it be exceptional in some way, or that it doesn't fall apart. Worrying about those things in my experience, usually just makes them more likely to happen and when they do occur it leads to fluster.
When things go to hell, they stay calm: Again this is linked to the previous two. Games can fall apart, things can take a nose dive hard in RPGs because it's in their nature. It includes so much freedom of choice on the part of the players, sooner or later most GMs find themselves caught off-guard or a step behind. And it is social game, so every once in a while, a GM can bomb just like a comedian can bomb. When that happens, the best referees seem to stay calm. This is a habit that has helped me out a lot. If you don't panic, it is a lot easier to get your footing. You can learn a lot of if you stay calm and just observe how and why the nose dive unfolds. It isn't the end of the world.
They are not perfectionists: This is a point some may disagree with, but in my experience the better GMs haven't been perfectionists. They know not every session will be brilliant, and they don't stress over that fact. Some of the worst GMing I've seen stems from striving too much toward perfection, believing if every session isn't hit out of the park, it is a sign they failed in their task. They doesn't mean they don't work hard, or they don't care, just that they don't have unrealistic expectations.
They are effective at resolving conflict: Good GMs know how to manage conflict at the table when it arises. I don't normally see much in the way of hostility or argument at the table, but when I do it is definitely a buzz kill. Because the GM is the one managing play, he has an important role here.
They enjoy GMing: Game mastering is challenging but rewarding. You have to put in prep before the game, and during the game the spotlight is on you to facilitate things. If you do not enjoy either the prep or the act of running a game, it isn't worth it and it will affect peoples' enjoyment. Good GMs like being in the GM seat and they like doing the prep work. If you are not excited about making NPCs, dungeons, setting material etc, then all these elements tend to come out flat.
They think about the game and gaming: When the game session ends, good GMs continue thinking about it. They ponder the possibilities, consider the ramifications of the last session and use it for fuel. They also seem to be the types who just spend a lot of time reflecting on gaming. This pays dividends in play, and is the prep work before the prep work. Before putting anything down on paper, it is a good idea to just sit and think.
They learn from their mistakes: No GM is perfect, every GM makes mistakes. The difference between the good and the bad, is the good ones learn from their errors. They look at mistakes as opportunities to improve. All of the really good GMs I know, have at least one story of a pivotal bad experience they learned from.
They don't realize how good they are: There are exceptions to this of course, but I find the better GMs don't know they are great, and don't spend time boasting about their abilities behind the screen. Maybe feeling like you still have room to grow, helps keep a GM in shape. Or maybe not being too big on the ego helps them focus on the game itself rather than their own personal sense of glory. I am sure there are boastful, arrogant GMs who are astounding, I just don't haven't met any.