The difficuly of vanilla didn't really come from mechanically difficult mobs or bosses. It really came down to invidual groups and how they played together
Say, if you had entry level group fresh from leveling and some questing, there were several key-things to watch
Patrols: Patrol at wrong moment could pull in extra group that could kill you, unless you had particulary skilled group utilizing their class to the fullest to pull it through. Had to remain attentive of surroundings all the time
Threat: Threat was a real thing in vanilla, and unlike cloudy memories of Lerk, it was actually far more involving with dance stancing, interrupts, stuns and even trying to AoE tank which was not easy as Sunder did cost quite the rage. On top of that, thanks to entry level gear, tanks had no hit so a lot of abilities could miss, making the choices you do a more hectic and threat you generate far less. One wrong miss sunder? Mob runs there, what now? Taunt miss? What now? Hope group is up to it. Or you know how to use rest of your tools which you had if you were good.
CC: Pulls were big and the mobs hit hard, one-two shotting clothies if taken unawares and leather not much better. You had to CC and keep eye on CC as they could often run out since mobs didn't tend to die in just couple of hits. Many mobs only were suspectible of certain CCs so either you had to play clever with short-term ones (stuns/incapatiates) or fit unconventional groups
Mana: Mana, especially on mentioned entry gear was very limited resource and the healer had to be really careful when to dispel and heal and especially whom, else having the threat of running OOM. Oftentimes players had to bandage because the mana just wasn't enough to go around. Healer also might've had alternative CC duties on top of that to keep track of
Trash abilities: Yes, these existed and they were nasty. Not in terms of "there's fire, don't stand in it" but "Fucker I just stunned you and I'm about to go and rape your healer". This created whole dynamic where players had to apply their class abilities to stop the mob, taunt the mob or anything. They ranged from stuns to fears that could devastate the group, to summoning extra mobs to simple lifedrain/boneshield combination that if not broken could up the mob entirely. Mobs also healed each others, those idiots. They didn't even need to be complex. A mob critting could gain enrage and then suddenly if healer wasn't awake proceed to take 80% of tank HP in couple sweeps. Tank was low? Ooops, too bad. Dead.
Bosses: Bosses weren't mechanically difficult but more so punishing in every little thing they did, especially dungeon end bosses. Good example being last boss of Strath UD. Does AoE damage to anyone within 30 yards, casts Shadowbolt taking 40% of tank's health, hits like truck, summons skeletons that if nto killed fast, heals the boss. Very, very hectic fight in itself to pull through even if not complex. Required stuns, AoE, offheals, interrupts and all nice stuff. Surprising amount of bosses were like that at end-game
It's combination of all these things that made the runs challenging, interesting and requiring attention.
The problem is that people later on who joined in got easier runs when they joined up with those already geared up to an extent, taking away a bit from the experience. Mobs dying faster so not being such big of a threat etc. But it was still there to the end at least until getting raid gear.
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