GnomeWorks said:
The world should feel alive. It's more immersive and more interesting, that way. The games should rightly focus on the players, but that doesn't mean that there aren't things happening elsewhere.
One of the best things in a well run campaign is coming back from A Trip Over Yonder and finding the world has *changed*. That bar wench now has a baby, and it looks a lot like you. Your grampa died of something no cleric can fix. The slimy advisor to the mayor...is now the mayor. Your pesky kid brother has got his first PC level and he's rarin' to go kill some kobolds.
Likewise, if you stop the orcs from invading the lands to the south, that means the overlord of the north can rampage unopposed -- and vice versa. There's forces in motion all around the world, and while you're powerful and skilled and all that, the world is going to keep turning whether you're watching it or not -- and then you have to deal with the consequences of your decisions.
To my mind, the quantum world where only what the PCs are watching exists is a boring one, one I can neither play in nor run. I expect my DMs to run a living world, where my actions *matter to the world*, but do not *define the world*; I try to give my players a place where saving the world matters because the world feels worth saving.
In my long running D20M Shadow Chasers game, part of the story of the PCs was their discovering how vast and expansive the 'hidden world' was. The discovery of organization, cults, agencies, and informal networks, of the fact they were one band of heroes among many -- not the first, not the last -- made the world believable, and their rise in NPC estimation from "Oh, great, another bunch of wannabe scoobys" to "OK, it's getting heavy. Call THEM." was the main arc of the campaign. They didn't matter to the world because they were the PCs; they mattered to the world because of the things they did.