While I have ripped Wyatt on ENW for his "guards at the gate aren't Fun... long treks through dwarven fortresses isn't Fun... skip to the Fun" - it actually makes some sense within the tight focus of the 4e system. The 4e DMG knows what it wants to do and is mostly pretty good about doing it - whereas the 4e PHB is pretty terrible at introducing players to the game (HotFL/HotFK much better, if anyone had bought them). DMG would still have benefitted from more clarity in explaining the kind of game it was, but the biggest issue was in presenting it as nominally another edition of D&D when its focus was so different from earlier editions.
I'm currently running both BECM Classic D&D and 4e D&D (and 5e D&D!) and they serve very different masters, they achieve 'Fun' most effectively through very different techniques. As you say, 4e works with hard scene framing, what I called Pemertonian on ENW, as it was Pemerton who helped me understand how 4e worked and why my attempt to run prodecural sandbox in 4e (conversion of Vault of Larin Karr) did not work well.
The 4e DMG hints at this difference, but IMO does not do enough to effectively distinguish 4e from its predecessors, given that most DMs will be carrying in baggage from 1e-3e. The DMG2 helped a lot, still without going all the way.
Instead of "X is Not Fun", the 4e DMG should have said "4e is intended for Y, not for X".