1. Knows The Rules
If you are going to GM a game, at least know the basic rules and have the rulebook handy just in case you need to look up something. I hate playing with GM's who have absolutely no idea what the rules are and refuse to learn them. While you shouldn't be a slave to them, the rules are there for a reason and should be used unless you have a good reason not to.
2. Doesn't See The Party As Advisories
Like I said in the previous post, the time that I have to roleplay has dwindled as grown older. Because of this, I'd rather play in a game where everyone cooperates to tell a cool story and have a fun time instead of one where the GM is going out of his way to kill us because he feels the game isn't fun unless he "wins". The only exception to this is when you are playing a "Deadly Dungeon of Death" as a one-night thing. Since the whole point of a "Deadly Dungeon of Death" is for the players to see if they can keep their characters alive in a dungeon where every room has something that can kill them. However, like I said, this should just be a one-shot thing and not the entire campaign.
3. Makes The Players The Stars, Not His NPCs
I think most of us at one point or another have played with a GM who constantly had his "badass uber-NPC" show up and save the players and steal the spotlight from them. If you're NPC is the start and is going to be doing everything, why the hell am I playing in the first place? The player characters are the stars of the story and your NPCs are there to be secondary characters. While you can have adventurers that have an NPC playing a major role, the player characters are still the ones in the spotlight.
4. Can Go With The Flow And Work With The Unexpected
As a GM, your players will regularly throw something at you that you weren't expecting and have no idea where to go with it. When faced with this situation, the good GM just takes a deep breath and quickly figures out the logical results of the player's unexpected actions. The bad GM will usually shut-down for a moment or two, then tell the players their actions have failed and force them to take one of the actions he planned for. Doing this really robs the players of any agency in the game and if done enough, they will start to feel like their actions have no meaning since they can never get off the railroads that you have laid out for them.
5. Doesn't Let Outside Biases And Prejudices Affect The Game
As I've said numerous times before, I play roleplaying games because I find them fun and I want to spend the free time that I have having fun. So, when a GM decides to bring his own personal biases and prejudices into a game, you can bet that I won't be playing with him for very long. I don't want to play with someone who's racist, homophobic, sexist, or hates something or someone for no other reason than its different from him and I definitely don't want to play in a game where he makes those things major parts of the campaign. I play this game so I can forget about the real world for just a little while and pretend to be someone different. I don't want my character to suffer because I choose to play a gay character and the GM just happens to be homophobic.
While these are numerous other traits that I could list, these five are the most important to me when I'm looking for a GM to play with. What are some traits that you look for in GM's?