I touched on this in my player archetypes post, but though it may seem obvious I feel I should elaborate on one point: DMs must be good players.
I'll touch on the most obvious points first: a DM who has never played is unfamiliar with a game with whose most intimate details he should be familiar to run a smooth and successful campaign. A DM not only has to know the rules well enough to run the game, but well enough to help his players, especially those who are new to the game. A DM who has been a player knows what information is vital to a player so that this information can be taught or given early on and the player is not instead frontloaded with mountains of information he cannot process all at once.
I've heard it said, and I find it a sound measure for a DM's success, that the only thing a player should ever have to know about the rules is what die to roll to do what he wants. The rest is, though often beneficial, optional.
A DM who knows his game well is able to take the weight of the rulebooks off his players' shoulders so that interactions can be streamlined to "This is what is happening, what would you like to do? ... roll a dX, add Y".
A DM who has played knows that flipping through books can cripple suspension of disbelief, since in real life all we have to do is see whats going on, decide what we want to do, and do it. Many players like to know mechanics, but those who simply want to act should be able to do so.
This is simply an example of the underlying principle behind being a good player: a DM who plays well knows the importance of his decision and style.
A DM who plays well also knows who he wants to play with.
The main point of my archetype post was to outline with what kind of players it is generally desirable to play. Well, no one knows that better than those who have played, and have experienced the good, the bad, and the ugly side of dice-rolling geeks. A DM who has played knows who he wants to play with, and can make this decision on behalf of his group, abiding by the Golden Rule in assuming no one wants to suffer through the same shouters and whiners et cetera that used to bother him.
A good DM-player knows what is fun and also how to tell when something is not fun.
When people are complaining that something sucks, no matter how much you loved the idea, it probably sucks. You'll think they'll hold out and be glad for the fun part, but would you have? Maybe, maybe not, but try to keep in mind. Most of us have played through good and bad DMing and know what its like.
In fact, very few of these points are revolutionary because they all revolve around the same idea:
DM as you want to be DMed. Our own iteration of a rule reiterated in every culture across all of time.*
In very brief, to save you a much longer post in which one would be hard-pressed to fit an idea that has not been stated before:
DMs, play your game. Play it a lot. Love it. Because if you don't, you shouldn't be running it. Why would you expect your players to love something you didn't? Unless you don't expect them to like it either, in which case you're running a deliberately mediocre game and you should never touch the dice again.
Take what you've learned by playing and don't ever forget it for a second while you create exactly the kind of campaign you want to play, and hopefully some other people will too.
Because if you have the talent, you should be writing the book you want to read, making the movie you want to see, singing the song you want to hear, and as a DM: creating the world you want to inhabit for a few hours every week. Anything less is choosing to do less than you can and if that's the case, why are you choosing to do the thing in the first place? Go ride a bike or pet a kitten instead.
A do it the best! Pet the crap out of that kitten.
Good luck and keep rolling.
*Check out Michael Shermer's The Science of Good and Evil for a great list of "Golden Rule"s across culture and time