I find most of the time when people complain about a game being too easy- they can't even complete the most challenging content themselves. Also, as someone whose first MMO was vanilla WoW, I cringe when people talk about challenge and what'll soon be 'classic' in the same sentence.
Vanilla's longevity was about wasting time, NOT about challenge. Outside of Naxx, which did have some actually difficult mechanics, there were few difficult fights- and think about the mechanics to some of the hardest. For example, Twin Emps. Mechanically the fight comes down to being able to switch tank aggro. Nowadays, those mechanics are considered basic- because games are designed to let you do what you intend to do without hassle.
And therein lies most of the 'time consuming' aspects of classic wow. Taking half an hour to get to a raid, spending another half hour waiting for single summons and single target buffs to go out- these are not in the least bit challenging, they didn't ever make for a good community- but they sure as heck took hours of raid time up. Instead of waiting in queue while you do other things, like quest or mini games or whatever- you stood in a main city spamming trade for hours hoping to get a group, losing party members multiple times and having to stop, hearth, repeat midway through if someone left.
That's what classic wow without rose tinted glasses was- waiting. Its content wasn't gated through talent, but extreme time requirements that resulted in raid days that were two to three times longer than what we have in games now. Gear was gated through having one or two pieces that had a 10-20% chance to drop, and had sometimes a dozen people who wanted it- think about that, it could take half a year to get a piece of gear you want. The most challenging part of every single raid (with some exceptions in the Naxx raid) was getting 40 people organized- meaning, ultimately, the challenge of raiding had nothing to do with mechanics or even what 90% of the players did.
Rotations sometimes were as simple as a single button plus the occassional cooldown- the most challenging part of my hunter rotation was making sure my AA didn't get broken by aimed shot, and following the tranq shot order. Most 'mechanics', like that tranq shot order, were not about reacting or thinking quickly in fights, but about following precise instructions at specific times, and the biggest threat was often randomized and out of your control- like unavoidable fears. If you had the right gear, and you could press 1-3 buttons, you were very close to being top notch efficient on almost every single spec in the game.