No one likes a rules nazi GM who wants nothing more than to look everything up in tables and roll dice (Roll 3D20 to turn the doorknob). However, like it or not, the rules are the mechanic by which conflicts are resolved, if a GM doesn't know the rules he leaves himself open for two things.
1) Players can take advantage of the GM's ignorance, blowing smoke up his ass about what their characters can and cannot do, turning the game into a terrible twinkfest, which is of course A Bad Thing.
2) The GM is forced to fall back on waving his dick around (I don't care what the description of that ability says, you can't do it period). This makes players unhappy, because the gameworld lacks consistency. Furthermore, players want to be able to use the abilities they've payed for - having a power just "not work" on a GM character is cheap.
I was in an Aberrant game recently where the GM wasn't up on the rules system - both rampant twinking and the GM dick were employed, and the game suffered for it. For example, I was playing a character with a Mega-Dex of 4. Mega-Dex 4 means you react so very quickly that you can literally catch bullets out of the air. Well, an enemy of mine shot a sort of web-gun into my face - I wanted to dodge but the GM wouldn't allow me, claiming that the range was simply too tight. In a normal game this call would have been right on, but in Aberrant a character with a Mega Dex of 4 could not only dodge that attack, but probably swiftly sidestep behind the attacker as well. Things like this make the player feel cheated.
Alternatively, I was in a D&D game where the GM was a god at D&D rules - he literally tore us to pieces. We had powerful high level characters that got ripped to shreds by a single, well-designed NPC that was entirely consistent within the rules. As a result, we were forced to innovate and come up with new strategies to counter our enemy.
Finally, a grasp on the rules system makes arbitrating conflict that much easier.
A GM doesn't have to be familiar with every nuance of the system - every exception, table, etc. However, the GM should have a passing familiarity with the rules system.
Rule 2: Know the World
This goes somewhat hand-in-hand with Rule 1. You should be familiar with the environment that the game system provides. The worlds of Aberrant and Warhammer Fantasy are markedly different - not only in their setting (Comic-book future vs. high fantasy) but also in the level of power the players have. Characters in Warhammer Fantasy are almost certainly going to die, whereas even starting Aberrants wield tremendous power.
I've played in Vampire: The Dark Ages games that degenerated into little more than D&D dungeon crawls. This is a perfect example of knowing the environment the game is suited for - a D&D style dungeon crawl in Vampire simply isn't fun or interesting. People play Vampire for the intrigue, the darkness, the morality play. Not the hack and slash.
Alternatively, I played in a Fading Suns game in which the GM was very familiar with the Fading Suns world - he was able to draw us into the conflicts, politics, and ordeals that characters in that world face. We benefited from his understanding of what works in a Fading Suns game.
Rule 3: Know Your Players
Not everyone is suited for every game. Some people prefer the casual, twinky fun of a good ole fashioned Cyberpunk game. Others prefer the moody tension of a well played Vampire scene. There are as many preferences as games.
Some players are more cosmopolitan than others - willing to give just about anything from Teenagers From Outer Space to Vampire to Deadlands a shot. Others are more narrow in their viewpoint - they like to play dark games like Vampire and Fading Suns, or they prefer combat-oriented games like Dungeons and Dragons and Cyberpunk. When selecting players for his game, a GM needs to keep this in mind.
The fact is, some players always want to fight. That's fine if you're running a combat-intensive game, but if you're trying to run a moody Vampire chronicle where the overt conflict is kept to a minimum, players like that may not have a place in your story. Likewise, a player who's fond of Vampire and gaining power at the expense of other PCs would probably not make the best choice for a good-guy oriented D&D campaign. It's up to the GM to be familiar with the preferences of his potential players, and choose the actual players accordingly.
That said, it's also the GM's job to ensure that the players in his game get what they want. A GM's primary purpose isn't to tell a story - it's to ensure that his players have fun (more on this later). Give the players what they want - not all the time, of course. But in the end, the players should go home satisfied with the game.
Rule 4: Know Your Characters
This rule goes hand-in-hand with rule 3 - the GM must be familiar with the characters in his game. A game which draws the characters together through common threads in their backgrounds and has references to those backgrounds (family, friends, old enemies, etc) is always more fun than a game in which the characters are cardboard cutouts of one another and can easily be replaced by any template character from the main rulebook. A GM must be familiar with character backgrounds and refer back to them to give the game a sense of consistency.
Furthermore, a GM needs to be familiar with the characters' abilities and weaknesses. Stories should be crafted so that both aspects of the characters' are exposed - their weaknesses and their strengths. Ideally the GM should pose situations to the characters that prey on their weaknesses, and the characters should have to creatively use their strengths to overcome the problem.
The GM should also provide situations that point out individual strengths and weaknesses - this fosters a sense of camraderie amongst the characters and is especially useful if your players are contentious. If one character is a combat monster, throw the group into a situation where that one character's abilities save the group from certain doom.
Rule 5: It's not your game
The game doesn't belong to you, the GM. It belongs to the players. The story is about the players - they shouldn't be parts in a larger epic that the GM is crafting, they should be the ones crafting the epic. A good game is a collaborative effort where the players and GM work together to create a story, not one where the GM creates the story and the players are along for the ride.
A common problem amongst computer game designers is Frustrated Writers Syndrome. In this situation, a computer game has an awesome plot, tremendous graphics, and is so amazingly linear that the player is just there to pull the trigger / cast a spell / swing a sword at the appropriate time. There is one way to progress along the plot, and that's that. A good example of a game that displays this is Metal Gear Solid.
This isn't a bad thing in computer games - the nature of the medium lends itself to this type of gameplay. However, for roleplaying games, this is *awful*. It's happened to me several times in games I've been in - the GM has such a clear idea of where the story is going that any deviation on the part of the players is quickly squashed, the players are only there to roll dice at the right time. If you want to craft an epic plotline to entertain people, you should be writing a novel or a screenplay.
GMing requires flexibility, the ability to work both with and against the players. If you aren't able to be flexible with your story or willing to adapt to unpredictable players, then you shouldn't be GMing.
A good rule of thumb is to design a game session with a sort of waypoint system - there are a few points the players have to get to during the course of the game (an important conversation that reveails part of the plot, a fight with a major villian, etc). However, the ways in which the player gets to these points ought to be flexible - there should be many ways to get from point A to point B. Furthermore, the GM should avoid letting the players know these waypoints exist - the GM must subtly herd the players from point A to point B, but the players should never realize that the points themselves exist.
Rule 6: Have Fun
If everyone isn't having fun, then the game isn't worth continuing. Of course everyone won't enjoy every session - sometimes people may be bored, other times people may be in a bad mood to start with. Sometimes the GM won't feel like dealing with it - that's all fine. However, overall a game should be fun. If it's not, then something needs to change.
Rule 7: Read
This is an addendum - read up on what you're running. If you're running a game set in NYC, then read up on NYC and actually use the setting. Know where the subway stations are, where the major buildings and landmarks are, etc. The authentic feel will help your players get more immersed. Likewise, if you're setting a game in the Dark Ages or a Medieval fantasy setting, read up on that time period to get a feel for how people lived, what the towns were like, etc.
Also, read a guide to writing stories - many of the ideas behind writing apply to GMing (But don't become a frustrated writer!). Characterization, foreshadowing, story structure, all of these things can help make your game more enjoyable for everyone.
Also, take notes - have a notebook (or better yet, a laptop computer) with information about your game ready so you don't forget things like NPC names, locations, etc.
Anyone else have any thoughts / comments on GMing? I'd be glad to hear them and add them to my repetoire.