I was there for all of those raids. I've raided in vanilla, in BC, in Wrath, and in Cataclysm. I've done hard modes and heroic modes since they were introduced. I'm neither the cutting edge progression raider nor someone who raids occasionally for fun -- I've been everywhere from a raider pushing for realm firsts to one leading a semi-casual 10-man while tanking. One thing I can and will say with absolute certainty is this: every single expansion to World of Warcraft has increased the complexity of the raid design.
Much of what makes modern WoW suffer the untrue label of 'easier' or 'dumbed-down' has absolutely nothing to do with actual encounter complexity, which has only gotten more elaborate over time. What makes it easier to raid is that gear is easier to acquire outside of raiding -- back in vanilla, if you didn't raid or devote serious time to PvP, as in played 12 hour days, you didn't get epic gear outside of a few random world drops or crafted epics. I remember when my night elf warrior got his dungeon set blues, that was a significant accomplishment to many players. The amount of time I'd expended running UBRS, LBRS and Strat/Scholo before that character even set foot in a raid was staggering, it was months upon months of game time. For a long while, Molten Core and Onyxia were the only options you had for raid-level gear, and if you didn't get a drop that week there was no other way to supplement that.
The only way in which World of Warcraft has become easier is that it has become easier to get the gear and step into some form of the raids. In terms of pure mechanics and fight complexity, the game is as hard or even harder than it has ever been. It's easier to play, in that you can get a smaller group together and go to a raid dungeon after running a few dungeons to gear up -- you can supplement your gear with valor points, run LFR and queue for content, things of that nature -- but the fights themselves are demonstrably more involved than the raids of the past. Even fights like the original Four Horsemen, Sapphiron or Kel'Thuzad from Naxxramas 40 (the hardest fights classic had to offer) were easier to explain to a group than some current fights. Their challenge came from the specific group composition, resists, and CC options needed to beat them.
Of course, it's hard to even define what you would mean by arguing that classic WoW was more difficult. Was it harder to tank back then than it is now? Harder to DPS? Harder to heal? Were the encounters harder? In my opinion all of these elements of the game have gotten more complex and demand more work to perform at their peak. Tanking often eschews the old "tank and spank" design, healers are often expected to watch their mana, DPS often must pay attention to several mechanics that will destroy them if they aren't executed. Compare this to Onyxia, a fight that had three phases, with one or two mechanics a piece. It took some guilds months to kill her, not because she was mechanically more complex, but because they simply couldn't put out the healing or DPS in the gear they had. I find the argument that it was harder because you had to herd 40 players to be disingenuous -- can you imagine trying to run Durumu with 40 players? Is the idea that we should return to the 40 player raid size? I can't imagine that being a good idea.
I played then, and I enjoyed playing. I wouldn't have stuck through MC, BWL, AQ and then Naxx if I didn't like the game. I even enjoyed raiding with 40 people, as horrendous as it could be at times to coordinate those raids. But let us not pretend. Let us not make the same tedious, factually inaccurate argument again. The raids were not mechanically harder. At times they were tuned so that current gear was insufficient for most players to kill them, and since the only source for gear was the very raids you were trying to clear, it became an exercise in frustration, but the mechanics were not harder that modern raids. They weren't even harder than BC raids, a charge many raiders were making at the time. Oh yes, players in BC did argue that BC raids were inferior in difficulty to classic raiding. Just as raiders in Wrath claimed that its raids couldn't hold a candle to the big fights of the BC raid era. And I'm sure you all remember hearing about how Cataclysm raids just couldn't match up to Wrath.
It's a silly argument. It doesn't hold up when you actually look at the fights and their mechanics. It ignores the way gear was acquired and how guilds approached raiding then. I am, frankly, tired of it. If we're going to complain about the game, which I know we all are (I do it all the time) then we should strive to make our complaints as based in reality as possible. This particular argument? It's absolutely absurd.