Here's some fun stuff I remember:
Warriors:
Vanilla: sword proc madness, instagib people left and right. 3k mortal strikes against people with 2900hp
BC: "Skillherald" - look it up if you don't remember
Wotlk: The brostorm era
DK's:
Wotlk: you could use /castrandom in arena and win
Cata: Blood DK's when vengeance worked in pvp. Hard counter all melee. 110k death strikes. Dancing rune weapon necrotics for 40k heal absorb. Yeah it was a gimmick and didn't work against mages, warlocks, or spriests, but still
Rogues:
Vanilla: the real stunlock. There used to be no pvp trinket and you'd die in an actual kidney shot not a blind vanish gouge smoke bomb repeat blah blah.
BC: combat rogues with mace stun. Legendaries
Wotlk: legendaries, FAN OF KNIVES OMG
Cata: legendaries
Mages:
Vanilla: double trinket pom pyro madness
BC: don't remember
Wotlk: shatter combos
Cata: shatter combos and 100 ways to freeze you
Priests:
Vanilla: Shadow priests hit... so... hard. Mind blast mindflay dead. They didn't really have any other spells.
Shaman:
Vanilla: Elemental shaman in tier 2 or higher. Double trinket, chain lightning, earth shock dead. period. Like quad boxing shaman, except this was One shaman.
Enhance shaman used 2h and a windfury proc was an instakill.
Hunters:
Vanilla or BC can't remember: pet enrage, solo clothies.
Wotlk: explosive shit
Cata: ???
Warlocks:
Vanilla: A brief and little known era with extreme spell coefficient on shadowbolt gave you 3k+ crits. Also, seduce+soul fire (there was no trinket so you were either undead, or dead)
others: several times affliction was dot and run. i mean really just run away and they died
Druid:
End of BC, End of wotlk insane bleed damage. Stacking cat bleeds and bear bleeds then running away
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