I cannot be called a casual player. I'm a Savior of Azeroth, heroic Dragon Soul-geared in my tank and DPS specs. I raid three nights a week. So when I'm telling you those dungeons were not tuned to be friendly for this experience, you can dismiss me as another casual player if you like, but you'd be wrong. Those dungeons were tuned to the point where you needed to have a cohesive group that knew one another and could either communicate quickly or use a method of communication outside of the reach of most PUGs. Especially in the gear that existed at the time. Various methods were attempted to balance this, including a stacking buff for random dungeon groups.
Now Nethaera explains Blizzard's reasoning on why heroic dungeons are rebalanced from time to time, and it's an argument that is easily extended to all content. I don't always agree with the rebalancing efforts Blizzard takes (my opinion on the Firelands adjustments was mixed), but I think the principle behind them is very sound.