I'm inclined to believe that it was deliberately left out. I get the impression that they don't want to tell you what's right at everyone's table, and leave that up to you to decide.
Some people enjoy that style of play, others don't. Who can tell them what edition they can or can't use that play style with? It's really nobody's business but those around the table.
KM, while you're not the only person to have expressed that view, I think it overlooks at least one thing, namely, skill challenges.The 4e DMG infamously includes a bit where it tells you that talking to NPC's isn't fun. The idea of many of the rules were "basically handwave everything except combat, and get very detailed on combat."
Skill challenges are a mechanic that manifestly is inspired by similar mechanics in other games (extended contests in HeroWars/Quest, scene resolution in Maelstrom Storytelling, Duel of Wits in Burning Wheel, etc). They are not about handwaving, but nor are they about combat. They are not mechanics that anyone would be familiar with simply from playing earlier editions of D&D. They therefore need guidelines and advice on how to set them up and run them. Part of those guidelines and advice have to cover issues of player vs GM authority, which play out differently in this sort of action resolution when compared either to handwaving (GM has overwhelming authority) or minis-based combat (very tightly mechanically constrained relationship between rules and fiction when compared either to handwaving or skill challenges).
To the extent that such guidelines were deliberately left out, it makes a nonsense of including those mechanics. It's a bad decision. It would be like including rules for calculating AC, and to-hit bonuses, but not mentioning the bit about spending a standard action on your turn to roll a d20 to generate a number to compare to the AC.
Another example of underdeveloped rules and guidelines: the 4e DMG has a paragraph in the chapter on adventures and quests pointing out the very great benefits of having players define the quests for their PCs. But does it anwyhere talk about the implications, of this sort of player-directed play, for encounter and adventure design (either combat or skill challenge)? No. The whole discussion of adventure design begins and ends with the assumption that it is the GM, not the player, who is in charge of defining quests.
The more frustrating thing is that it's not as if 4e offers anything new to RPGs in these repsects (although some if it is new to D&D). There are plenty of examples of good rules text and guidelines to look to. (And in DMG2 they got Robin Laws to cut-and-paste some of it from HeroQuest 2nd ed into the D&D rulebook, but didn't bother to do the extra work of translating stuff that works for the HQ mechanics into stuff that would work for the somewhat similar but by no means identical 4e mechanics.)
Rules Compendium and the Essentials DM book are, if anything, even worse. If all I had read was those too books, I would have no idea how to set up and run a skill challenge. At least with the PHB, DMG and DMG2 I get given enough information to try start making translations across to the decently-written rules that I have in my HeroQuest and Maelstrom rulebooks!
Bottom line: I think it's a mistake to publish a game whose attractions, in comparison to many other mainstream fantasy RPGs, include a more modern approach to player-GM relationships, player role in story creation, and getting the mechanics to work as a game as well as they work as a device for engaging the fiction, but when it comes to writing the rules and guidelines to assume either (i) that everyone already knows how to play this new sort of game, or (ii) that everyone is going to play your new game just like a slightly crappier version of your old game (because it won't support that older style of play so well) unless they're read enough rulebooks for other games to see what your new design is actually aiming at.