Wrath heroics in the beginning, were still pretty hard, but not many remember them cause a lot of people came to the game during Wrath. You used to do Vault( the wintergrasp raid) in all blue gear, even in 25 man. But as the expansion went on, epic gear became easier to get, badge gear, Trial of the Champion 5 man gave great gear, and people were blowing up old heroics. It really wasn't so much that Wrath heroics were easier, as it was people outgeared them pretty quick getting Vault and Naxx gear, and the later gear just made it that much more AoE fest.

The problem with the Cata heroics was that they were too hard for those that didn't know the old ways of doing heroics. They wanted players to use abilities to CC mobs like they did in BC, but too many new people didn't play during that period. And you add in the fact that the Cata Heroics didn't drop a single epic quality item, was a real downer, cause for those that were playing since at least BC, at the very least the last boss of the heroic should drop an Epic quality item. Didn't matter if the heroic blue that dropped would have been the same as the Epic one or not, people felt cheated going through all that hard work just to get more crappy blues. Perception's were already created, and they tried to change them, they lost a lot of people because of it.