For me on mage, it was on the PoP, where you can charm elemental for several mins without charm breaking. Charm one of the earth looking elemental and take it down to the Stormrider area and bi*ch slap them around, HAHAHA!!!!!
Original PoP era, "Pet Canni" for mages. With the right focus (cast haste, conjuration cost reduction), AA, and Specialize Conjuration a mage could summon a pet and then use an instant-cast Reclaim Energy item to sac the pet for a good bit more than it cost to cast. Combine that with a mount and mages had unlimited mana for nukes in groups.
I got a fun one for you Ion. Back in original 1999. On pvp servers. Charm and fear COULD be used on players. I have so many memories charming a player and running them into a swarm of orcs in EC. This was patched away less than half a year from release. Here are some patch nots that prove it could be used. Couldn't find the patch notes removing it though. http://www.tski.co.jp/baldio/patch/19990607.html
Not being able to illusion through locked doors. On the other hand, that nerf caused me to start a rogue alt that is now level 120 and led me to box for the first time. So, a mixed bag for me.
Dire Charm nerf for Enchanter. Lost quite a few Enchanters on Cazic-Thule when they couldn't rip Plane of Tactics apart with a perma-charmed mob for endless AA and TS items anymore. Then the subsequent charms nerfs throughout the years reducing damage, etc. PBAE Stun nerf. This was April of 2021. While it probably didn't matter much on live, it probably killed a lot of AE caster group fun on TLPs, I bet. Those were good days on old-school Live when I got to experience it at least.
Enchanter mana crystal used to give more mana than it consumed. It wasn't by much like ~600 mana, but you could chain it to have infinite mana. It'd just take a lot of time.
The biggest nerf was clearly snakes losing the ability to kick. Snakes need sweet ninja kicking action.
Here's a couple more of the many chanter nerfs: They took away Lurch, and our only snare (it was an AA with virtually no cool down. It mezzed a mob, and when the mez wore off, the mob was snared. So AA mez + any DoT = snared mob.)
Not a lot of pet classes used it, but the Aegis line used to block any amount of hit so it was great against named monsters. 2015-01-22 18:33 Changed Slot 1 from "Block Melee or Spells (9)" to "Block Melee or Spells (9), Max: 18918"
Being able to mass TL while in combat. The original implementation of /pick zones: - Once you hit min number of players required TWO pick instances would spin up WITH chance for names to spawn! - Our guild was extremely behind in gear after TBM and used the picks to gear up mains =/ ... And the single biggest nerf ... CH chains !
Being able to cast multiple levels of the same DoT spell on a mob. This most hurt enchanters who have only one DoT line. Now, they can use just one DoT and do the best they can with DD type spells.
30 second recast added to enchanter self rune. Effectively eliminated rune tanking which was hella fun back during The Darkened Sea era.