Disclaimer: This is not, and should not become, a "Vanilla was better/worse than current content"-thread. It's just a small thread to share some nostalgia and maybe get a few smiles out of people who played then.
With all of the "zomg vanilla suxd/vanilla is teh asom" threads I always notice one problem: They rarely focus on the small stuff. Stuff that made the game annoying or fun.
The only thing you ever hear about is how classes were so imbalanced then and raids were so great and had a feeling of achievement to them.
So here's a thread about small things that got fixed along the way but might still give those who played back then a smile, or get some headshaking from newer players.
I probably won't remember everything correctly because it's been more than 6 years now. So please correct me on that.
Negative Resistances: Early in the game it was possible for players and NPCs to have negative resistances. So you'd do extra damage to the affected target.
Out of combat rezzing: There were specifically designated people that did not engange in combat with a boss. This way the did not get put into combat and could rez people during the fights.
Paladins and Shamans being faction exclusive: This often gets shoved under the rug, in vanilla there were no shamans on alliance side and no paladins on horde side. It was the cause of alot of grief for both sides.
Unlimited flask/potion buffs: It was possible to stack any amount of flasks/potions you wanted. The distinction of battle/guardian elixir also didn't exist at the time.
Buffs not stacking: HoTs did not stack back then. You could only have one instance (per rank) of spells like Rejuvenation/Renew on a player.
Auto pot: There was an addon that automatically let you chuck a potion whenever you reached a certain amount of health.
Respecing was expensive: This hasn't technically changed, but 50g was a very high amount of money back then. The amount decreased with every month you didn't respec, but it was still too high for most people to regularly respec.
Limited debuff slots: At first there were only 16 (correct me if I'm wrong) and later 32 debuff slots. And that was in 40 player raids. Warriors in our guild were not allowed to spec into deep wounds because of that.
No shared trinket cooldowns: You were able to use both trinket slots at once. Arcane mages became infamous for the "ZHC+ToEMP+AM" nuke.
Weaponskills being unlearned upon respec: Aside from the fact that we even had weaponskills back then, ENH shamans ran into the problem that the always had to level up their 2h weaponskills if the respeced out of and back into enhancements.
Wind fury procs: It was technically possible to oneshot somebody with a WF proc that procced WF that procced WF and so on.
Percentage instead of ratings: Ratings came first with BC, before there were percentages. The Eye of the Beast was one of the best trinkets for casters back then. Because it gave 2% crit.
No multiple flightpaths: Without addons it was not possible to fly directly from Orgrimmar to Silithus. You had to take every single step to there manually.
No AoE buffs: I'm not just referring to stuff like totems or LotP being partywide only. Oh no. There was no spell called Arcane Brilliance or Gift of the Wild. Instead Druids, mages, priests and especially the poor paladins, had to buff every person in the raid individually. If you were a priest you had up to three different buffs to give.
And here's a category just for mages, because I played one back then and remember more about that class than the other ones:
Rolling ignites: Every stack of ignite from every mage was rolled into the first ignite placed on the target. And the damage of those didn't disappear individually, instead it reset the timer each time a new one came up.
Arcane Explosion had a casttime unless speced into it in the arcane tree (which almost everyone did)
We only were able to summon 4 of the highest level water. So mages had to appear early to summon water for the entire 40 man raid.
It was possible to have different ranks of the managem in your inventory (which also shared a CD with the Healthstone from WLs)
Dampen/Augment Magic: We had two spells that increased/decreased any heal/spelldamage done to a target. It was rarely used, but still entertaining.