Mercurius
Legend
I'm pretty sure you'll find a variation on this one in many RPG book introductions, but still. . .
'Bang bang. You're dead.'
'No I'm not.'
Actually, its not this. Rather it is the indeterminacy factor--the rules, especially dice rolling, ensure a sense of danger, of chance. Even if all the players know that the DM can pull his Fiat out of the garage at any time, they still have to roll the dice, they still have to avoid missing three saving throws after they go below 0 HP (one of my favorite idiosyncrasies of 4e).
In some ways the whole D&D experience is centered around rolling a die...I mean, making a crucial attack roll when the party is almost down and out...isn't that what D&D players live for? But the story, the context in which the die roll occurs, is what brings it to life. So my point is not that you have to choose between mechanics and imagination, not at all, but that mechanics should serve imagination (and story), not the other way around.