As sort of a devil's advocate position here, how about the following scenario?
- A game, with no level scaling, allows the player access to Dungeon A, Dungeon B, Dungeon C, Dungeon D, and Dungeon E. There's no hard enforcement whatsoever on which dungeon needs to be done in which order, but each one is progressively more and more difficult than the last, in alphabetical order; the player will become more powerful after completing each dungeon.
- The game's player is pretty good at the game, and likes a challenge; the most fun that he or she can have is by starting with Dungeon D; beginning with A is only barely tolerable, B and C are still kind of bland, and E may or may not be doable, but is a little excessive for a first dungeon to tackle.
Scenario 1: If he or she
does start with Dungeon D, then the only dungeon that's
really left is Dungeon E. A through C were too easy to begin with, and now that the player is even more powerful from completing Dungeon D, A through C are going to be actively, aggressively
unfun due to sheer lack of difficulty/engagement. Should he or she move on to Dungeon E, then A through C remain, and that problem is even
more severe.
Scenario 2: Conversely, if he or she starts with Dungeon A, then the entire game is
tolerable, but only just so; by the time he or she reaches Dungeon D or even Dungeon E, the increased power from slogging through Dungeons A through C are going to make D and E just as boring.
Neither scenario really strikes me as a winner.