Here's a quick one I was thinking about. I've seen a lot of DMs put great effort into their descriptions. And there are certainly moments where that is appropriate. There are scads of books on good DMing all of which usually tell you to be really thorough in your descriptiveness, and how important it is to describe things in detail.
I am going to suggest that when it comes to descriptiveness, often less is more.
Here's why: RPGs are a shared experience, not a shared world.
To illustrate: In one of my campaigns, one of my players was running a character who was blonde. He saw him as blonde haired and blue eyed. At one point he mentioned this, after months of the game being run, and not one of the rest of us, myself (the GM included) had realized that. He had mentioned it originally upon character creation, but for some reason I, and everyone else in the group, had imagined him either brown haired or dark haired.
Meanwhile, he'd gone along merrily imagining himself blonde. And it did nothing to impede the running of the game.
The fact is, contrary to what the narrativist Swine out there might think, we are not playing in a common world. Its a shared experience to be sure, but to waste time in forcing the players to see everything the same will actually bog down the game and make it unpleasant.
One of your most powerful tools for making an RPG campaign enjoyable is the player's own individual imaginations. You might be able to imagine some pretty cool things, but usually nothing will be as cool TO THEM as the way THEY imagine things.
No matter how much effort you put into describing your sinister dark wizard, nothing will work better than, say, picking one cool detail ("a scar that runs down his face, right over a milky dead right eye"), and then leaving the rest intentionally vague. "You see the dark wizard Bargle, in black flowing robes with arcane symbols. A scar runs down his face, right over a milky dead right eye".
That's it. That's all you need. The PCs will fill in all the gaps.
So when is great detail required? Only when a particular detail is important beyond a merely descriptive sense. When its actually important to the game, either in a tactical sense, or as a clue, or because its a critical macguffin for solving the crisis.
Otherwise, trust me, less is more. Harness the power of your player's own imagination.
(originally posted October 22 2005)