The Attunement requirements for Burning Crusade's 10 Man Raids and higher ruined the end game content.
And they were tuned, then removed, during BC (and don't exist in Wrath of the Lich King at all).
While the 40 Man Raids were difficult, often requiring Guild Partnerships, attempting to attune 25 people through to the Black Temple was a bad joke.
Attempting to get people sufficient gear to run Vanilla Naxx was a bad joke, since you needed 39 other people to go run an otherwise obsolete raid. Now you can get up to one tier behind using 5 mans, which means you can pull someone into a raid and have them be useful without having to go farm the previous raid tier.
The preparation Grinds extended some 400 hours for a single run through. That never existed in Vanilla WoW.
No, you just had people farming level 40 dungeons like Maraudon and the only available to one guild a week world dragons for Nature Resistance gear in order to do AQ40. Certainly no grind there at all. And of course there was no grind in the massive amounts of consumables used in high end vanilla raids (which was also fixed in BC).
Combined with the shift in focus to Arena Combat that Blizzard implimented, meaning Battlegrounds were now filled with AFK Honour Farmers, meant that both PvP and PvE were left desolate; provide 10 hours a day to Grind to prepare to run the End Game content, or Grind games for Honour to get gear to be attuned to then grind 10 hours a day to prepare to run the End Game content. The PvP aspect devolved into "lose faster so we play more games of honour" and the PvE aspect devolved into "grind to prepare to grind for reputation to gain access to the grind for attunement so you can grind to prepare to run a dungeon".
... you mean the PvP system was bad, in comparison to the old ranking system where the only way to get to Grand Marshall was to have other people play your account so you could do the necessary 90 hours a week of grinding battlegrounds? Or the battlegrounds that were great right up until a team of Naxx geared players showed up and obliterated everybody because there was no way outside of raiding to get competitive gear? PvP players got shafted in vanilla, and as much as I hate arenas for other reasons, that was the one thing they did fix. (Cataclysm has done a better job of that with rated battleground play.)
Sorry, but vanilla was significantly better than that.
Unless you were a Paladin or Druid who wanted to actually use that tanking tree Blizzard gave you. Then you learned that it was only there for pretend purposes, and raid tanks were Warriors. Period. Or you were a Priest, Paladin, Shaman, or Druid who wanted to use your DPS spec in a raid. LOL! Shut up and heal.
Saying that vanilla is better then the expansions requires the worlds most powerful pair of nostalgia-tinted glasses. Or for people who liked addons that could play the game for you. Those were removed in BC too.