It's a mixed bag. It's a really grand plot and I enjoy the really huge scale that I'm painting in on some level, but the down sides are pretty huge.
a) That really huge scale gets hard to achieve with the same level of concreteness that I'm used to working in. Lots of the time I'm not sure I'm conveying the living world of the game like I should be because I have to be abstract and wing it just because the canvas is so large.
Here's a hint - the canvas you actually work on doesn't have to be large. It can be small, so long as the stakes of decisions are large.
Take my session last night (which is Deadlands, not D&D, but the principle is general):
The PCs are not known for restraint. Their motto seems to be, "We are good at intimidation, the dark arts, and sustained violence". So, I set up a situation where thoughtful restraint is really a good idea, to see what they'll do with it.
In essence: The BBEG has created a kaiju monster. He was able to do so because the PCs dropped the ball some time ago. If they don't stop the monster, the BBEG is going to turn a cold war into a hot war, and things will go to bubblegum shortly thereafter. The PCs are informed that their usual weaponry simply isn't going to cut it. They discover that one way to beat it is with another monster.
They rescue a crucial NPC from the BBEG's clutches. I take some effort to make the PCs become attached to the NPC - he's been through torture because the PCs dropped the ball, and is nearly a broken man. But, in order to make their own monster, he must go through another ordeal. If the NPC goes through this ordeal, he'll come into his own full holy power, and be able to help the PCs make their own monster (a *friendly* one), and then the NPC will go off and deal with other problems in the world, as is his destiny.
So, the ordeal comes up - it is a hostage situation, the bad guys have a firebomb, and will destroy the orphanage the NPC grew up in (and kill all the kids there currently) if the NPC doesn't give himself up. The PCs then proceed to try to deal with the problem all on their own. The NPC remains in tow, but doesn't get to do anything to help resolve the situation. He feels no particular stress, has no challenge or ordeal.
The PCs *accidentally* trigger the NPC's transformation - the mad scientist PC, seeing no way to quickly defuse the bomb, decides to pick it up and throw it out the window, thereby setting off it's motion-sensitive trigger, immolating himself. While this was not the nominal way to get the NPC to come into his power, but seeing someone willing to risk self-sacrifice seemed to be appropriate to me. Holy power suffuses the NPC, allowing the PC to be healed (lucky him, as he'd have been dead otherwise).
But the canvas upon which the characters act is still small, personal. The real issues are matters of individual choices - the world is saved because one PC was willing to take a huge risk on himself to save a dozen children.
b) Because the PC's are convinced that they need to save the world, they are pretty much on rails.
That's true. Once the PCs perceive a threat that large, they're kind of obligated to deal with it, unless there's a clear path to hand it off to someone more capable than themselves.
The ways I've managed this is twofold:
1) I didn't *start* with things that were themselves immediate threats to the world. I started with small stuff, each of which fed into the larger threats, but no one of which was key. They have time to interact with the world, and the starting threads, for a long time before saving the world becomes the things. During the time, the PCs have a lot of freedom to do what they want, because dropped balls aren't themselves major issues.
2) The BBEGs are taken from the PC's backstories.
These two lead the players to be thoroughly invested in the action. They may be a bit on rails, but they *want* to solve the problem, as it is personal by the time they realize the world is at stake. They don't really *want* to do side stuff - this is the most important thing to them.
