4E doesn't have an ongoing, supported setting.
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it also has a kind of default vibe or atmosphere, a "meta-setting" that includes the new planar structure and the mythology that's been presented in the "theme" books (e.g. Underdark, Plane Above, etc). But it doesn't have a setting that is supported in an ongoing way, that is being explored and developed through supplements.
In this respect 4e resembles a game like The Dying Earth. I've never read the Vance stories, but feel that I could run a game of Dying Earth from the rulebook. It gives me the "vibe" and "meta-setting", plus tips on how to set up situations/scenarios that will exploit that vibe to produce a fun session.
My feeling is that 4e was written with the intention to be GMed in this sort of way. I say this because (i) it fits with the game's emphasis on the encounter - combat or non-combat as the basic unit of play; (ii) it fits with the obvious effort to create that default atmosphere, with the gods, race backgrounds and so on in the PHB and the little sidebars in the Power books; (iii) when you look at the original MM (with most of the campaign info located in skill check results), plus think about how skill challenges should play out (with the GM having to make calls about NPC responses, and other elements of the gameworld, on the fly in response to unpredictable player actions), and even look at the whole emphasis on "situations" rather than "world exploration" as the focus of play, the game seems intended to support "just in time" creation of world details, using "points of light" and the default atmosphere as a framework for doing this in; (iv) it fits with the absence of a developed setting.
Unfortunately, though, the rulebooks don't do much to support GMing this sort of game. A contrast is provided by The Dying Earth rulebook, which does offer tools to help the GM with this sort of situation-based preparation and play.
There is also the ability of a campaign world to be a source of ideas for a new DM or player. Good art, good stories, and good descriptions can all be catalysts for creating a character or an adventure, or even be the core of an entirely new world.
For 4e, this is really provided by Worlds and Monsters. Good art, interesting stories, and (most importantly for a GM) good discussions of the way in which those stories have been designed to help make an interesting game.Big chunks of this book should have been incorporated into the 4e DMG, in place of (what are in my view) unnecessary or overlong parts of it like the tedious discussion of giving adventure locations personality and the random dungeon generation. If they had been, that would have gone some way - though not all the way - to helping GMs run games in the sort of fashion that the rulebooks seem to intend.
I tend to read the complete AP's over and over again in preparation for running them and the Paizo crew really does give you a very good feel of whatever part of the world the AP's are taking place in. There's a lot of good background stuff there that yes, some of which the PC's may never know or find out about. But a good DM will make the material his own and find a way to disseminate that info so that it's relevant to your players. Not all of your players will retain this stuff, but for the ones that do? You have something with which to feed their curiosity and interest in the world.
For better or worse, this is the type of play that 4e does not seem to have been designed to support (although later books like The Plane Above, MM3 and Monster Vault are heading in a somewhat different direction).
I'm not saying its not a factor at all- those two clearly feel it was- but for most non-adopters, it was issues of marketing and mechanical/fluff changes they didn't like that kept them away from day one...IOW, LONG before lack of setting support could even be considered to be a factor.
I tend to agree with you
and with Mercurius, because for the reasons I've given I think that the lack of a setting
isn't a coincidence relative to the mechanical and flavour changes, but rather fits with them as part of a coherent (but, as it turns out, perhaps not so popular) overall design.
When 4e game out, I posted on these forums that WotC apparently agreed with Ron Edwards that a narrativist-oriented RPG focusing on situation and character-driven play would be more popular than a simulationist RPG focused on the players exploring the world and/or stories that the GM creates for them. Such a belief seems the only way to explain the presence, in 4e, of all the features I've mentioned above.
At the time I tended to assume that WotC weren't just speculating but actually
knew- unlike Ron Edwards, for example, they have marketers and market researchers on their payroll. But it seems they may have got it wrong.
For someone like me, who wanted a game like the one they produced, it's turned out to be a lucky error. The tone of Essentials, though, plus the release of Nentir Vale, suggests that WotC might be pulling back, and trying to turn 4e into a more traditional RPG.