I just had a discussion with a group of people and we where debating what it takes to be a GM. We figured you need 3 things:

1. Role playing: this involves making the world come alive, this might also include humor if appropiate to the group and game setting (good-“You crest the verdant hill overlooking the castle and are assaulted by a wave of nausea caused by rotting flesh”, bad-“ You, um, like see a castle in the distance, but there are like all these dead bodies so you feel sick”)

2. Plot: this involves creating an interesting story as well as maintaining a good pace (good-involves an interesting story that keeps the players wanting to play, bad-simple story, cliché, utterly predictable, silly story).

3. Action: you need to have good balance and appropriate challenge (good-the fights are challenging, exciting, fair and fun, bad-the fights are annoying, impossible, boring, dull, etc).

When we game usually either I or one of my brothers GM (most of our games involve our friends and relatives, we play a wide range of RPGs). Personally I think I’m rather bad at step 1, about average at step 2 and pretty darn good at step 3 (IMHO). Actually when I GM often many people just want all action with a little story thrown in, well at least I get a 1 out of 3. My younger brother doesn’t GM much, but he’s ok at 1, pretty good at 2 (I have to admit), and a little below average at 3. My older brother used to GM the most, but now doesn’t live nearby, he was pretty decent at all 3 steps (though I’m better at action and my younger brother is better at plot). Anyone agree/disagree with these steps, have something to add? Any GMs out there willing to grade themselves. Actually this made me realize that I need to work on my role playing, live and learn.

GMing is a lot like making a movie. In that case, you see movies have the same values as those 3 items.

However, the MAJOR difference between movies and RPGs, is #2. You sit and watch movies. It's passive.

If your RPG plot is all planned out, you're having the players watch a movie. A good GM will have a dynamic plot that adapts to the players actions, and not vice-versa.

That’s a good point; maybe I should emphasize that and add a fourth section:

4. Improvisation: the ability to change your game in reaction to the player (good-you can react dynamically to player actions, bad-you railroad players into doing what you want)

5. Managerial Skills: the ability to manage a disparate group of people with different tastes, desires, and expectations. This applies not only in game (game styles and preferred character types), but out of game as well (personality types, different cultural norms).

Basic math skills/Access to a calculator

-Because your players sure as hell won't

-Tir

Well these might be bonuses rather than necessities, but I reckon it would help to have some of the following skills as well (although all might fall under Canis's #1):
Writing
Acting (ideally with a range of voices & accents!)
Art & Graphic Design
DJ (nothing like having the perfect tune to fit a mood)

The ultimate GM would be good in all these areas, I reckon.

Ability to read thoughts and predict the infinite possibilities stemming from a single actionn - yeah, it's something like that.