Metagame mechanics and the generally-accepted idea that the details of the fiction shouldn't have any effect on action resolution.
Mainly this. The "dropping of the pretense" that 3E and earlier simulated much via their mechanics was also big.
Prior to 4E, there was a lot of instances in D&D where you had a mechanic that produced one effect, and the fluff said another. For example, not infrequently the fighter was portrayed in fluff as more scary than he really was, strictly mechanically. I'm not as bothered by that discrepancy as some people are, as player attitude about the character matters a great deal to the experience. But it would be equally unfair to say that the discrepancy never caused problems--sim, game, and story.
Also, on action resolution, I'd qualify "effect" as "direct effect". Whatever simulation is left in 4E (or "emulation" if you prefer) is very consciously of the indirect type, frequently more macro than previously. That is, in 4E you often still get "simulation" of
results that are in tune with the fluff, often more so than previously.
A party of adventurers goes into a dungeon and finds a dragon. Claw, sword, spell, and fire breathing are involved. Characters and monsters get hurt. What you frequently end up trading in the simulation department is that you drop the idea that the fighter mundanely swung his sword a bunch of times, hit the dragon, and gradually wore him down. You add that the fighter mixed in some rather exotic moves with sometimes dodgy direct cause/effect simulation explanations. OTOH, you also add that mechanically and within the fluff, the fighter got to stay in the dragons face, replacing the discrepancy where the fluff said he could, but the mechanics didn't support it.
If the discrepancy was something that gave your DM wiggle room to play around with the narrative, and he or she used it, and it didn't much bother anyone at the table--then you lost some simulation capability. If the discrepancy was something your DM didn't care to exploit this gap, and the players didn't want to deal with it, then you also lost some simulation capability--that you didn't really much value, because the discrepancy irritant was too great for what was delivered through it.