You seem to be treating flavour as mere colour. I don't see why it should be treated that way.
Because by definition, that's what it is.
For instance, a 1st level Basic D&D PC cannot move through, or break down, a dungeon wall (at least, not deploying the standard resources of that system, including typical tools in typical ingame timeframes). Yet such a PC can clearly move through a forest, or for that matter burn it down.
Yes, but, so? The point is that however they choose to move, you hard scene frame them to the next room of the forest. The scenario is similar to an adventure game where however you go left from one screen, you still arrive at the same screen. However the move, you treat them as if they were 'in the corridor' between rooms. This is in practice a very common technique. I've observed DMs run campaigns that had a single room in the entire game world, what you might call 'the stage', and whenever the party moved they just changed the drapes and brought new players on stage. Every movement in the game was simply a hard scene frame where the players declared their intention to change the scene. It's not bad, I do it myself at times for urban adventures where a map would be too large and too complex to use. Running a 'wilderness' as I described is only one step up from that.
I another thread I talk about how I don't yet have good rules for burning down things of any size or scale. Yet recently this happened in my game anyway. How did I do it? Fundamentally I just changed the drapes. Since there was no need to interact with the fire, it didn't matter. A forest fire also need not have any meaning for the story. Maybe I'd have some NPC now go, "You ruined my drapes, you vandals!" (in so many words), but that's about it.
What do you mean "the way you relate to the game doesn't change"?
Back to my example of the 1e campaign, prior to the change to the paragon paradigm we related to the world primarily as heroes. We spent our time doing heroic things, like going on quests, facing monsters, defeating evil foes, and rescuing the innocent. When the campaign changed it caused us to change the way we were looking at things. At first we kept trying to do heroic things when they came up, but we soon realized that not only was it usually a waste of our time, but it was unnecessary and kept us from our ever mounting list of more important duties. We weren't heroes any more. We were rulers; we had heroes. So instead, our henchmen and retainers were sent to do heroic things, and the reported back to us. They were our scouts and agents, and we only needed to step in personally when they reported that things were over their heads. So the main campaign became mostly about ruling, directing, and so forth, and when we were dungeon crawling or questing as a break from the main action it was often us playing a party of our main PC's henchmen. We had become those important NPC's that earlier in our career had been sending us out on missions, and we knew then why they weren't doing it themselves. They were fighting larger wars, often several at once, arranging grand alliances, and seeing to the needs of the literally 1000's of NPCs whose life, health and prosperity depended on them. Battlesystem and mass naval combat rules (which we had to invent) were more important to our main NPCs than dungeons. We counted retainers among our most important treasures. Victory was less about preserving our lives than it was keeping NPCs alive. Our characters started having marriages and children. Our character sheets included maps of the castles and other properties we owned - we didn't invade 'dungeons' nearly as often as we defended them! Our perspective had changed. It was a campaign unlike anything I'd played before or since. Sure, you could do this sort of high political game at any level. But it became the game we needed to play at a particular level, pushed in that direction by the rules (if in a primative way) by the rules regarding strongholds and henchmen.
I had been forced to leave it before it before the campaign did end or wind down, but I understand by the end of it at where the characters were 16th-18th level, it had grown to where it involved a magical 'nuclear' interplanetary war with fleets of flying ships of the line bombing flying cities. Had the campaign continued much past that point, I suspect that it would have needed to grow again. Perspective would have shifted. We weren't rulers anymore. We were demi-gods; we had rulers. You can see that shift occur near the beginning of the Wyre story hour.
For many campaigns, the way you relate to the game world doesn't change. As you get more powerful, you foes become more powerful, but the fundamental challenges you face and the methodology for facing them doesn't change. The day to day conduct of the game or the metagame doesn't change. I see absolutely no sign in 4e that its expected to.
Why are these any more epic than, say, "The world is on the cusp of a new Dusk War, in which the world may be undone (and perhaps rebuilt) by elemental chaos, or rendered into perfect stasis by construction of the Lattice of Heaven, or [insert whatever other possible outcomes are salient to the campaign in question], and the players - via their PCs - will determine which outcome eventuates"?
Because in general, when you are at lower than the epic tier, you are determining how the world will change only in the negative or in a reactive way. That is to say, "The world changes" is generally defined in some way as defeat enacted on the PCs by someone else - undone by elemental chaos, rendered in perfect stasis by law. These are changes to the world being driven by some truly epic being, and the role to the PC's is generally to just stop them in some fashion. This can be seen in the current campaign I'm running. My protagonists are currently heroic tier (6th-7th level, with a couple having political rank 3 on my scale of 1-10). Thier quest is to stop a 17th level necromancer from doing something (exactly what they don't yet understand because they are still locked in their survival oriented view of the world). But the 17th level necromancer is epic tier. His quest is to change the rules of the world and the game, which is epic. If he suceeds, I'll have to write new house rules for the setting (which in this case would be the same as new options in the system). But the option of making new rules for the setting is currently something that the PCs (or players!) haven't even thought to aspire to. All they are aspiring to is to prevent change from happening (and stay alive doing it). That's not epic... yet. I have built in possibilities of the PCs going epic once they realize they can.
And in practice those changes you talk about never really happen, because either the PCs 'win' or the game doesn't continue. Rarely do you see a campaign written with the PC's trying to bring about a new meaningful change in the world rather than stop it.
Upthread you talked about "the rules of the game system". Now you're talking about "the rules of the setting". These are different things...
Yes, I know, but they shade off into each other. A change to the game world like, "No pure blood elves.", or "No clerics.", constitutes a change to the setting. Exactly where that starts to become clearly a change to the system as well isn't clear, but I think at some point it clear is both. The point is that the change is large and tangible and meaningful.
I personally don't understand why that is more epic than "You just killed Torog. Now the elemental chaos is not being held back from the underdark, and hence the world is threatened with dissolution."
a) What you just described is just flavor; what I described wasn't just flavor.
b) What I described meant the players had been the active agent. What you just described just means that the players are again in the reactive role - trying to stop something rather than do something.
c) What I just described involes a tangible change with immediate impact. What you just described means nothing. The world is always 'threatened with dissolution'... in the background, as a drapery, as a trope to move the action and give it the illusion of urgency. Nothing is actually happening. Now, if instead of 'threatened with dissolution', the setting acquired a new rule - "Any object which is left unattended by a sentient being must make a Fort save every 24 hours or disentigrate", and as a result you actually had the world disolving and the players had to arrange elementals to watch them sleep lest they wake up in an empty abyss, that might be epic.
To some extent I don't disagree with this. But I think that the attitude you seem to associate with epic play - for instance, that the players via their PCs are responsible for driving the game and making fundamental choices - is something that I favour as part of play at all tiers.
No. That's your words and your understanding of what I'm saying. The PC's are always the protagonists regardless of tier and are always driving the game. What changes isn't merely that the PC's more and more drive not just the game but the whole setting, though that is important too, but thier perspective on it.
The whole point that was made, by myself and other posters, is that epic is not about bigger numbers but about changing scope of the story.
While I agree, I think that changing scope where scope only means the drapes is probably even less interesting and less important than changing the numbers. You can see changing numbers but not scope in something like the 70th level wolves and bears in World of Warcraft. But changing the bear model out for a demigod model doesn't make it for anything more than changes in the flavor. It's the illusion of scope - nothing in WoW actually has scope except the developers (and can't because its a persistant multiplayer world, so all PC driven changes are meaningless). Taking that further, if I take a 5HD Ogre Magi, change his club to the wand of death, change him to appear to be Orcus, change his lair to a bone palace in the Abyss, and then have the players invade the bone palace to defeat Orcus and save the world, the scope and scale hasn't really increased any more than if I had made the Ogre Magi 30HD. It's changing the drapes on the room. If I do both, 30HD orcus in a bone palace, I still haven't really done anything meanful.
In Queen of the Demon Web pits, you kinda can see the attempt to change the scope and scale for real, but they don't really succeed. Ultimately, it's just a dungeon crawl and the elements with potential are left to the DM to develop.
You see the same problem again in Temple of Elemental Evil. There is room for more scope and scale in the Elemental Nodes, but in actual fact, they are left only as simplistic dungeon crawls. Both and Q1 end up being completely unsatisfying to me, as you can see from the threads where I suggest how to rewrite them.
Now, Chronicles of the Dragonlance actually does change scope and scale - from Heroic to Paragon. It's got problems and is very difficult to run well, but it has scope.
Most of the adventure paths I've read by Pazio, while in many ways quite good, don't change scope (much). Ironicly or not, maybe the one that comes closest is the one that they wrote for evil PCs, though admittedly I haven't read Kingmaker so I can't judge.