I was at Naxx and I disagree with you.
It was not that hard, raiding today is not hard either. What made Vanilla so tough was simply the fact that you needed a very big group of players, that were ok with sitting out a lot or passing on gear.
On my realm we had a MT, at AQ 40 he was the first fully decked char on our realm, he was having BIS gear, was spoon fed pots and flasks…
At Naxx, he got an offer from another guild on the server, he left the guild he was with from day 1 and they could never recover from that loss. Their best geared tank, was just gone and the number 2 was not bad, but lacked the gear.
One player can screw a guild or boost it and I am afraid with today menthality, this might happen even more. Needless to say, that at Vanilla, we did not have 1.12 talents, we had 1.1. and did raid with that. We did not have 16 debuff slots and upgraded items. All of that, created a tough environment, plus the rep grind and general lack of knowledge about the game.
Did you need to be some no life hardcore guy to play and raid Vanilla? Nope, but you needed luck, a lot of luck. Losing the MT, or some healer could be GG and push you back months in progress. It was a common thing that guilds with sponsors stole well geared tanks and healers from smaller guilds, not only to claim the world first spots, but also to have a huge amount of replacements like with the feral for Patch and Huhu.
If such a top guild never recognized ferals, then they just stole one from a smaller guild. The smaller guild was pushed back and could no longer progress.
The mechanics itself, are just as easy as today, just that they were new and at times less predictable, like at MC or BWL with certain boss abilities.
In my opinion, the biggest issue with Vanilla 2.0 will be the guild system and if people are motivated to play a month without any upgrades, because that is what Vanilla is. You farm flowers and food, get rep and the reward is nothing unless you are lucky. I doubt that today instant gratification community will be up for that, they will not wipe on boss mechanics, but they will on their own lack of stamina and willpower.