AbdulAlhazred
Legend
Well... If you look at the doors, they are described as being composed of different, and progressively more rare and magical/tough, materials as the DCs increase. This is just like the terrain. Thus, IMHO, fiction is entirely driving these things. Were an adamantium door to appear in a location where level 1 PCs happened to go, it wouldn't magically be a DC15 door to break down. It would be DC40 (or whatever it is) and the level 1 party would just be SOL if they tried to break it. The fiction wouldn't present that as a necessary option, obviously. No more than it has to present traveling through meters of solid rock as an option to a level 1 PC. Some things ARE impossible.This applies to doors too. Which means that the "objective" lists in the DMG are no more than advisory, just like the monster stats in the MM. This is why I say that, ultimately, 4e is "subjective" DCs: if a player thinks that the DC doesn't match the fiction, the issue isn't that the GM has wrongly estimated the DC (as it would be in 5e, or Burning Wheel, or another "objective" difficulty game) - it's that the GM has failed to persuasively convey the aesthetics of the situation.
I think this is a big part of what makes 4e so robust in play.
Likewise a level 12 bugbear is the Bugbear Gung Fu Master of all bugbears, or somesuch. Clearly if you decide to mess with that bugbear, such will be telegraphed in some fashion such that a low-level PC won't ever mess with it, assuming for some reason they even appeared together in the same scene (possible).
I guess what I'm saying is, I don't take 4e's 'environments of a given level' shtick too far. The world isn't divided into zones of strictly different levels, or races of strictly different levels. There are trends, and there is fiction that can happen which presumably has room for the players to pit their PCs against the appropriate obstacles, while other elements serve some other story purpose, or perhaps just exist as wallpaper, maybe hinting at some opportunity or threat that could factor in later (ala DW Fronts).