Not so much “why I don’t play that class” as “why I don’t play that spec”
Retribution Paladin - back in cataclysm I found myself forced to play this godawful spec called retribution just to do normal questing because someone thought it was a great idea to remove all combat abilities from holy Paladins… it took me over a month to figure out the damn rotation so that I wouldn’t run out of mana and could keep my holy power rolling enough to actually attack… That experience has soured me to the spec ever since…
Feral Druid - where to start… should I start with having zero character identity? I mean seriously, I play an MMO with character customization, and you ask me to play a spec that says “your customization means nothing”… hard pass… but wait it doesn’t stop there… not only does my character customization mean jack diddly squat as feral, but I’m giving up my customization to basically play an entirely different class instead… if I want feral’s play style I can just go play Rogue.
Guardian Druid - see feral… replace rogue with warrior…
Affliction Warlock - DoT… DoT… DoT… wait… DoT… DoT… DoT… wait… I would like something more engaging than this… it is boring and tedious… two things that you never want to combine ever… like seriously I didn’t even think it was possible to make a class in a game that was both boring an tedious, let alone one that could cause boredom induced headaches while playing…
Arms Warrior - great big two handed weapon melee build… awesome… just charge in and kill things with overwhelming power right? Wrong! It is more like corpse run simulator… damage is abysmal, heavy armor means jack apparently, it’s only self heal is unusable in actual combat unless you talent for it, and even then you lack the damage mitigation to survive it’s cool down.
Fury Warrior - Alright, so Arms clearly fell flat on its face… but maybe doubling your ability to wield massive two handed weapons will double your power and make you an actual force to be reckoned with… and… corpse running again… turns out you still lack any semblance of survivability… also, “requires two handed weapons” really ticks me off when I find some great one handed weapons that I’d love to use for transmog… who’s bright idea was it to make titans grip not only baseline but a core requirement of the spec?
Balance Druid - I loved this spec for the better part of WoW’s life… I even suffered through its many awkward incarnations because I wanted to see it be as awesome as it could be… but despite all of that… it became quite clear during Legion and persists into BfA that unless you had the exact perfect gear with all the right procs and bonuses, you would always be out damaged by Restoration… WTF! Why does the healing spec have higher baseline damage and scale better in general than the DPS spec? Seriously WTF blizzard!
Assassination Rogue - … … … DoT… … … ok I thought Affliction was tedious… now let’s add combo points into that same tedium… and put you in melee… with a weak HoT for a self heal that has a long CD… oh and swap mana for focus… as if you didn’t have enough to manage…
Outlaw Rogue - Can I just have Combat Rogue back? No? Fine… I’ll try this new thing… don’t get me wrong… I love the flavor and lore for outlaw… it is certainly more of a rogue spec than combat was from a lore and flavor perspective… but mechanically… it is a hot mess… there is far too much RNG to this spec to be realistically reliable… you might get a 10 second streak of fantastic luck when the stars align and everything dies as you turn foes into sliced Swiss cheese or you may get 90 seconds of rotten luck and become a walking pincushion… assuming you live for the full 90 seconds…
Holy Priest - when it comes to group content I almost always play a healer in one form of another… but this one… I have never wanted to pull my hair out more as a healer than when trying to play a Holy priest… it doesn’t matter what I do, I have tried following guides everything… people die faster than I can heal them as this horrible spec… I can play a Discipline Priest like it’s nothing though and rarely see people dying… the only other healer that ever made me want to pull my hair out was Mistweaver monk but I would rather play a Mistweaver monk for a solid year non stop than ever play a Holy priest again…
Havoc Demon Hunter - unlike everything else on this list, I actually have nothing against this spec… I just prefer Vengeance is all…