Gamemasters have some pretty amazing tools and resources available to them. I know people who are into d20 3.5/4.0 have a number of programs for remote play, drawing up tactical maps, tracking character development, and managing combat rules and sequences. I'm not so much talking about those things. In part I see those things as symptomatic of some of the things I don't like in games. Running for thirty years or so I've learned some things about what fits my style of play and gamemastering.
First, a GM has to have many skills-- they have to be able to craft a story, they have to be able to improvise, they have to be able to articulately express and convey emotions and details. All of those things I'd consider sort of higher order skills-- or maybe they'd be better described as creative or expressive elements. There are also a set of skills a GM needs that might be considered more meta-- understanding player needs, managing different personalities at the table, managing conflicts, and assisting the group dynamic. Those are worth looking at in more detail some other time. But a GM needs to have a handle on some other, more basic skills.
The obvious thing here would be the grasp of the rules and the ability to craft/write up/point up adversaries and villains. But for me that's really a secondary consideration. A GM should know the rules of the game, but not to the point of bogging down in them. They should be able to make a reasonable interpretation on the fly and, if they get it wrong, be willing to fix it later away from the table. Some genres require a more crunch heavy version of this, like Supers or DnD (shudder). I've been moving away from this over the years.
I don't write up all the stats on my bad guys. I don't because it is a waste of time for me. What I need to know are general ranges, some interesting details, and three to five things they can do that make them different from any other baddie. if they're Mooks, I just need one or two details. If a combat goes well, it ought to be done in about an hour (unless it is a big fight). In that case, I don't want to spend more time than that in preparation. There are exceptions-- big bads, special events, climax sessions and so on. I'm more likely to put more detail into super-villains but that's the nature of that genre.
The thing is, as a gamemaster, I have all the points. I can kill the players anytime I want to. Any GM can do this, regardless of who they are. it is part of why I don't get GMs who seem to have an antagonistic approach to their players: they can always win. Now some argue that the GM needs to play fair-- they need to have monsters of an appropriate 'challenge level' or have complete stats for all of the villains. I had a player who was convinced if the GM didn't have that, then they were cheating. I don't buy that-- whatever I do at the table in terms of combat should be done to serve these purposes (not necessarily in this order):
-Provide an exciting and action filled scene
-Allow players to use combat based abilities and stretch their creative abilities
-Let players roll lots of dice
-Give them the satisfaction of a concrete victory (or alternately raise the dramatic stakes by having them fail in some situations)
-Advance the plot
I'm not saying combats need to be cakewalks-- far from it. More satisfaction comes when there's something at risk. The GM just needs to be creative about what they put at risk, how they dole out consequences, and what would make for the most cinematic scene. Cinematic in this context can mean many different things-- for a horror game that pace and pressure is going to be very different than one which apes Final Fantasy.
So spending too much time writing up all of the very specific stats of bad guys is not an effective use of time. It is more effective to think about how to make the setting of the fight cool, coming up with some great possible 'moments' for each of the baddies, and developing a way to make the players feel like you as the GM is really going to hose them. Because, again, killing them is easy. Injuries, temporary effects, loss of equipment, loss of powers, debilitating wounds, collateral damage...all of these are more interesting.
I'll come back to this tomorrow to look outside of combat about what marks effective use of GM preparation time (for me).