Cataclysm has succeeded in reintroducing mana as a resource for healers. How much mana your healer has is a major factor in whether or not your can win a fight. Because of this, dps paying attention to encounter mechanics has become important, because if they take more damage than they have to and the healer has to keep them alive, then the healer is more likely to run out of mana.
Our first few heroic runs were good demonstrations of this. I was drinking every two trash packs and our ability to beat the boss was very tied to my mana pool.
Three days later, I was never drinking and the idea of running out of mana on a boss was laughable unless we really, really blew it.
The difference was not that our dps got so much better at handling the encounters. My guild members are pretty competent, so when it became clear that we had to actually do things right we did things right. The difference is explained by how encounter survivability works.
In Wrath, healers didn't really think about mana and their heals were powerful enough to heal the tank to full very quickly, so tanks didn't worry about avoidance. All anyone worried about was whether the tank could live through a large burst. Survivability was pretty much all about the tank's health pool. Because of this, it scaled linearly with gear.
In cataclysm, the tank is not in danger of being bursted out because a heal can land unless they make a mistake with encounter mechanics. That means that avoidance is quite relevant since it decreases long term damage. Since it takes multiple heals to move a tank from low health to full health, the power of the heals is very relevant. Since it is plausible to run out of mana before the end of the fight, and more mana means the ability to cast more heals.
Instead of survivability scaling roughly with the stamina of a single character, it scales with avoidance * healing throughput * mana. In other words, it now goes up with gear cubed instead of linearly.
Adding 13 item levels was enough to turn heroics from a significant challenge into something pretty close to a cakewalk. Another tier of gear and heroics will probably be a daily chore to get Valor Points.
This is the formula that explains pre-Wrath gear check bosses. A gear check boss was one that was not that difficult provided that you had sufficient gear, but that became completely impossible below a certain gear level because your tank would simply die or your healers would run out of mana. Wrath had gear check bosses as well, but they were bosses that required a certain amount of dps gear to beat an enrage timer, not ones that relied on healer and tank gear.
These two types of gear checks are not mutually exclusive. Last night we downed our first Cataclysm raid boss and he had a tanking/healing gear check though the damage he did as well as a dps gear check by way of an enrage timer. Fights like this are extremely easy to overgear and trivialize. Of course it was a very simple fight, and the second boss we attempted seemed significantly less simple.
I hope that as we go forward with Cataclysm bosses we find fewer gear sensitive bosses and more skill sensitive bosses. So far I've seen an even split.