I've given up being a DM with 4th Edition. I've mostly been DM for 20+ years, but now that there is an implicit animosity to world builders I don't feel the desire any longer. As to the new Cosmology, it's badly conceived but with good writers could be ok; but if they start shoe-horning every old setting into it they will lose me completely. I bought the 4E FRCS and skimmed it once. I doubt I'll ever read it again. Even bad 3E books got a few reads from me, that's how bad I feel 4E is to read.
Hmm, different strokes for different folks I guess. 4e fired my imagination like nothing since 1st edition AD&D has, and I developed a world based off the 4e rules without noticing any impediment or antagonism on the part of the rules and books. 4e reminds me much more of myths, legends, fairy tales, and ghost stories with its background, tone, and themes - much like 1e AD&D. 4e dropped a lot of the tie-in of rules and fluff of 3.x, and the rules of 4e are MUCH more modular- I can alter something in 4e or ignore rules with almost no consequences, in contrast to 3.x where even small rules tweaks in the interest of flavor for a setting could snowball out of control. I also find leaving the baggage of the Great Wheel and its silly alignment-coded planes out of the new edition a much welcomed change. The Feywild, Shadowfell, Elemental Chaos, Astral Sea and Far Realm are far more evocative to me than Arcadia, Olympus, Tartarus, the Negative Energy Plane, or the Abyss of previous editions. Finally, IME 4e stands in the way of roleplaying less. I run RP-heavy simulationst games, and I felt 3.x tried to put far to many mechanics into RP aspects of the game when none were needed. Roleplaying is not "I ask the duke to let me lead a force of 100 soldiers against the orcs. I rolled a 28 on my Diplomacy check." To me, the player should make a case and roleplay out the situation rather than relying on die rolls as a crutch. 4e handles this better mechanically via skill challenges (for those that want to use a mechanical method for roleplaying) by making the number of successes achieved have a result along a continuum of possibilities, rather than the binary effects of social interaction rolls in 3e (either the duke lets the PC lead the soldiers, or not). As for the simulationist aspect- I want my fantasy world to make sense, so I figure out detailed histories, trade routes, political struggles, religious wars, monetary/cashflow/resource issues for countries, and NOTHING in 4e prevents or hinders me from doing this.
In contrast, I felt 3.x forced me into a corner with world design because of the heavy interaction of rules and implied fluff, magic levels, and class balance (caster dominance). I found the 3.x rules were actually a rather severe impediment to world building, because it forced me to build with rules as a primary consideration, rather than history, theme, tone, and interesting ideas as the primary concerns.
In any case, play whatever works best for you. My group and I are playing 4e, and never looking back. But for folks who like 3.x, there are literally truckloads of material out there, and Pathfinder is still around for you, so we all win.