Looking back to classic raiding
First things being first, let us look at the argument being put forth. The assertion is that although the numerical difficulty of raid encounters has decreased -- the DPS and healing requirements being easier now than they were in the past -- the mechanical difficulty of these encounters has increased. Because even the most basic raid encounters have a higher mechanical difficulty than previous bosses, this sets the bar extraordinarily high for new raiders. In past raids, players were capable of making far more mistakes; also, there were fewer mistakes to be made, and these mistakes were not as costly to the raid as a whole.
It's quite a bit to take in all at once, and sadly, there isn't a simpler way of bringing it up. We'll break it all down in parts and address each issue separately to compensate. To begin, we have to look at the foundation of the argument, that today's raid encounters are more mechanically complex than past raid encounters. Depending on how you look at things, your view on the matter could be relatively skewed one way or another.
Comparing any current encounter to one found in the original Molten Core is a pure disservice to this discussion. I don't mean that in the sense that it is rude; it is more that one must consider the limitations of the time. Players had far fewer tools, both via in-game skills and now common addons. Blizzard had far more technical limitations and was inexperienced in raid design. A large factor in the difficulty of the original Molten Core (and all of vanilla raiding) was more the massive amounts of farming involved to get the gear and items required to stand a chance at the encounters, more than the mechanics of the encounters themselves.
This is not to say that Blizzard did not create brilliantly complex encounters during classic. Indeed, once they had the experience of Molten Core and Blackwing Lair under their belt, the design team churned out a wide variety of highly complex encounters: C'thun, Skeram, Rajaxx, the original Venoxis, and then a vast number of encounter in Naxx. What is important to note is that Naxx was reused as the first raiding content of Wrath, and many of those encounters still stood the test of time.
The Karazhan complex
A popular comparison for this theory is to look at either the encounters or numbers from Karazhan and use those as a comparison tool against what we have seen thus far in Cataclysm. This is a rather flawed view, as the data provided by Karazhan does not provide a full picture. Karazhan was Blizzard's most successful attempt at making raiding more open to a broader playerbase, but it was never intended to have the ramifications that it did. Blizzard understood the accessibility problem long before The Burning Crusade. This is why it created Zul'Gurub and the Ruins of Ahn'Qiraj, the first attempts at getting more players involved in raiding.
Karazhan, however, was unique in many ways. By many standards, it is a widely popular instance, but one thing that players fail to realize is that Karazhan didn't fulfill the purpose that it had. Karazhan was meant to be an introduction into raiding, a springboard for players to take a leap off of into the rest of what there was to offer. Yet very few players actually managed to do this. If you were to compare the numbers of players who cleared all of Karazhan and then moved on to actually clear Gruul's Lair and Magtheridon, it would be significantly lower than the population of players who raid today. Many players never progressed beyond Karazhan, and fewer still were the players who continued what progression they did make. As you moved from Gruul's on to SSC, then TK and from there, Hyjal on to Black Temple, you saw a pattern of fewer and fewer players who managed to complete these raid instances.
While a large portion of this is attributed to the increasing difficulty of these encounters, part of that was also due to the raid structuring of the time. Nowadays (and even back in Wrath), raids are designed to become outdated as each successive raid is released. Back in vanilla and The Burning Crusade, you had to gear through each raid in order. A raiding guild could carry you through higher content, but a fresh group of players would have to raid starting all the way at Karazhan and move its way up. Now, you can gear primarily though 5-man content in order to be prepared to enter the highest tier of raiding.
Spanning into Cataclysm
The data isn't yet there to support Cataclysm raiding, and even once Dragon Soul is released, we still won't have an entirely accurate view. Blizzard has taken many great strides to reduce the difficulty of raiding content while still attempting to keep it challenging and engaging, especially for hardcore players. With the past nerfs made to Firelands in order to open the content up even further, the data that we could glean now would be vastly skewed in favor of the most recent content -- yet that in and of itself is half the point.
Raids have been more mechanically difficult than was in the past. This is absolutely true. While the problem is as great as one might think, it is the inevitable nature of the game. Players gain more abilities, more sophisticated addons are created, and the newest content has to reflect this. Yet despite it all, none of the mechanics that we see are anything that surprisingly new.
Alysrazor tornadoes? Heigan dancing. Omnotron? How many council-style fights have we had so far? One in Ulduar, Black Temple, Gruul's Lair, Naxx ... and that's just off the top of my head. Debuff juggling, add catching, AOEing, teleporting, no healing! We've run the gamut of what this game can throw at us. Encounters might be getting more complex, but players have the tools to deal with them. Complexity does not directly correlate to difficulty.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. Be sure to look up our strategy guides to Cataclysm's 5-man instances, and for more healer-centric advice, visit Raid Rx.