Type A: Challenging the Strongest - This type of raid encounter is characterized by the individual challenge not being the same for every player in the raid. A typical example would be the simple tank'n'spank raid boss with lots of health, lots of damage, and few or no special abilities: In that case the challenge falls hardest on the tank and his healers, whose skill and gear is essential for success. In a type A raid encounter, at least some players have a large margin for errors. If somebody makes a stupid mistake and dies right at the start, that doesn't necessarily cause a wipe. In fact the raid encounter is beatable with less than the maximum number of players in the raid, so everybody else is just an extra. This allows the raid group to bring weaker members to gear up, or even sell raid spots.Basically the problem with Cataclysm heroics is that they are type C hard, challenging the weakest. A good example is Baron Ashbury in Shadowfang Keep, because he is so bad that he gets nerfed in patch 4.0.6: He has TWO self-heal abilities that must be interrupted, and if not everybody gets their interrupts times exactly right and coordinated with each other, it's a wipe. You can actually be in a group which is completely unable to kill him, if you get not enough classes with interrupt abilities. But there are a lot of other examples, and even on trash mobs in Cataclysm heroics everybody is required to play better, e.g. using crowd control correctly.
Type B: Challenging the Average - In a raid encounter of this type, the raid as a whole has to come up with a defined level of performance. A typical example is raid bosses with an enrage timer: The raid has X minutes to deal Y million of damage to kill the boss, or they wipe. Thus you can easily calculate the damage per second that the raid has to deal on average. That does not mean that everybody has to deal the same damage; it is possible for some very good players to compensate for the lower damage of less skilled or geared players, or even a single early death. Nevertheless a certain minimum performance would be recommended from everyone, because several early deaths or complete incompetence from too many players would make it impossible for the raid group as a whole to get to the required level of performance.
Type C: Challenging the Weakest - In this type of raid encounter special boss abilities are used which result in errors of any player causing a wipe. Usually this is done with some sort of debuff, which requires a fast reaction from the victim to not hurt the whole raid. As the debuff is random, the raid group cannot afford to bring anyone not likely to react fast enough, as that would cause a wipe for everyone.
Note that players generally consider that the overall challenge of a raid encounter goes up from type A to type B to type C. But in fact the difference is mainly affecting the weakest players in the raid group. For the strongest players there is no inherent difference in the degree of individual challenge in the three types.
And that is the problem. Wrath of the Lich King heroics worked well as PuGs not just because they were easy, but because the little challenge they had was of a type A or B, where the tank, healer, and best dps could easily carry the inevitable bad dps player or two. That doesn't work any more in Cataclysm. And this especially regards dps classes, because even those who didn't let themselves get carried through WotLK PuGs spent the last two years training themselves on optimized damage rotations, often including AoE, and are barely aware that they have crowd control abilities. A "good" dps in Cataclysm is one who knows about crowd control and aggro management, not the one on top of the dps meter, which basically requires them to relearn their classes completely.
Now in spite of what the elitist jerks say, World of Warcraft has more decent players than bad ones. But in a type C 5-man group your chance to succeed equals the chance that a random player is good enough to the power of 5. So even if you'd assume a high percentage of 80% good players, the chance of success of the group is only 33%. If you believe only 50% of players are capable, the chance of the group to succeed goes down to 3%. Because everybody in the group has to be good, the success or failure depends on the weakest player, and the chance to have not a single weak player in a random pickup group is slim.