The main thing that usually gets brought up almost instantly is the difficulty. Back then there was no Raid / Dungeon Finder, so if you wanted to raid or run dungeons you had to find a group of 5 for dungeons, and for raids 40. It’s hard enough to get 20 competent people at the same time every week to run mythics in legion. Can’t imagine how much of a pain in the ass it was to get 40 people together to raid in vanilla.
2. Class Quests
At level 10, hunters had to go out and tame 3 different pets before being able to tame a “real one” for themselves.
Warlocks had to do lengthy quest chains to summon their demons, then defeat them before they go to summon them.
Shamans had to run all over the world to get their totems
Rogues had to level up 2 class specific professions to make their own poisons or to pick locks.
These are just a few examples of how big the class quests were in vanilla, pretty much all classes had quests that needed to be completed in order to get “basic abilities”
3. The Grind
Everything was a grind, quest items routinely had low drop rates. There weren’t enough quests to get to max level, so you had to grind mobs for those last few levels.
You had to go out and grind weapon skill levels everytime you got a new weapon
You had to grind any and all gold you got since there was no way to earn it passively
Many raid fights required you to have resist gear that you had to grind
Getting the High Warlord title in pvp meant full-time job levels of grinding pvp all day everyday
In vanilla wow mobs hit hard, mana pools were low and default regeneration was really bad. Some classes had some kind of Regeneration mechanic tied to getting Killing Blows on mobs, but no matter what class you played you needed to have food on you at all times. After killing 5-10 mobs your health or mana would need to be replenished.