Again, though, that's great to say but, when the rubber meets the road, system matters. Say your group has chosen to enter the "Simple Dungeon of 6 encounters". Bog standard dungeon crawl. It's nicely non-linear - multiple paths are possible and there is no "end goal" other than whatever the players want to achieve. Nicely sandbox right?
But, here's the thing. Most people can't do that on the fly. You have six D&D encounters. That's probably around 10 different stat blocks, each of which is very detailed. You can't do that on the fly. It needs to be prepared. And that takes time. Often, quite a lot of time depending on the level of the PC's.
Now multiply that by a hundred in order to be able to allow enough player choices to count as a sandbox. At the high level of simply describing things, that's easy. But, in actual play? That's a MOUNTAIN of work.
As you know (at least I think you do!), I, of course, agree that system matters enormously.
What I was trying to point at was a generalized approach to general content creation rather than a specific approach to specific content creation (such as a 6 room dungeon for challenge-based priorities).
If we're talking about building out a dungeon crawl for challenge-based priorities, we really need to index the particular system we're talking about because the process and the demands of pre-play prep vs improvisation are each going to look different depending upon the system. Some examples of this are:
* Moldvay has a particular procedure for prepping dungeons. Map w/ rooms having multiple ingress/egress points, theme, stock w/ denizens/puzzles/secrets/loot, build out Wandering Monsters table. Torchbearer (unsurprisingly) is similar.
* But D&D 4e and Dungeon World both work very differently than Moldvay or Torchbearer. Both the concept of dungeon and the execution of the play will be very different.
For 4e, you should probably be organizing the "delve" as a Skill Challenge with each decision-point sending you deeper into the depths on a success or generating some kind of complicating twist/escalation (including nested Skill Challenges) on a failure. 4e can trivially handle this on the fly.
Whereas Dungeon World is going to be a different beast entirely. You might have a Front for a dungeon with particular moves, Grim Portents, Impending Doom. Or you might just have an idea that spawns suddenly out of immediately preceding play and you just put relevant dangers/opportunities in front of the PCs and let the the snowballing move resolution engine generate a cascade of situations that coalesce into a dungeon crawl as you go. As with 4e, this will be on the fly, but it will be a very different affair then 4e in terms of the feel and execution of the play.
Regardless, neither of these two paradigms of 4e or DW delving looks nor plays like Moldvay or Torchbearer nor is such delve-play prep-centered + tradition crawl procedures. So yes, I certainly agree, you very much have to index what game you're GMing (and playing) as you consider the variance in task before you.