My opinion is m+ followed closely by the azerite system then the covenenants as they were when we started SL.
A lot of people would say garrisons, but I actually liked them, in fact, really unpopular opinion here, after I could fly, I rather liked WoD and wish they had actually finished it.
Semper Fi! 
Pathfinder and anything remotely like it…
/wipes tears from eyes
Does the WoW token count?
Mythic Dungeon in general were a bad idea. Even M0.
It was better to have Heroics with the special Hard Mode achievements with special circumstances to be met (take no damage, avoid the xyz, etc…)
They won’t even put Mythics in the LFD tool.
Hard gear resets every season.
This all day long, this is the only system they’ve ever implemented that made me stop my sub. A sub I had mostly left intact with the oddball month off during college beans/rice budget since TBC pre-patch because I farmed like mad for 4 months straight to get my worst legendary and immediately saw my raid invites fall off a cliff. That sub was dormant until end of BFA (last tier).
pretty much every single increased grind system implemented from WoD forward
all they cared about was that cash shop gold revenue kept going up
Always amazed how much some raiders hate on M+. If it wasn’t for it I would have quit and played FF14. M+ is the one thing that game doesn’t have and the only thing keeping me playing WoW.
Do I have to name just 1?
Pathfinder: They took out flying in WoD, got pelted with unsubs, then put in Pathfinder as a compromise. It stopped you from just buying flight at max level, to where you’ve already seen the content and done it on foot/ground mount. It made no sense why they changed it.
Loot Lottery: In BFA, they made it to where you could run LFR and get Mythic iLvl gear, and then people thought at that point that gear > skill. It messed up a lot of groups.
The Alliance.
Lok’tar Ogar! Oh, and MOOooooOOOo!
If you don’t recognize realm names, now I’m sure you haven’t been paying attention. You should at least recognize realm names as realm names.
The system tries to put together groups that contain multiple players from the same realm. It actually succeeds in getting more than 2-3 players from the same realm more often than random chance would suggest.
It never puts players into full groups of players from their realms. If that’s what you’re waiting for, it’s not going to happen.
If you want to put together full groups from your realm, try spamming trade chat. Queues would rarely pop if you had to wait for hours for each group.
The only compromise was between the dev department and the accounting department, which told the devs to do whatever it took to stop people from unsubbing. A true compromise requires two parties negotiating in good faith, not an ultimatum.
They changed it to get more playtime out of players while providing less content, requiring repeating trivial content endlessly.
That doesn’t sound right.
all the Shadowlands systems, soulbinds, covenants and ripcords, It was that the borrowed power was worse than in each expansion that we would not have wanted to have been borrowed, and we would continue with artifacts weapons indicating the power that our characters as in role would indicate that they reached the maximum power.
I meant to edit, but accidentally hit the trash icon, very sad.
There was a concept called Titanforging in BFA. Which would let people “win the lottery” that would increase that armor’s/weapon’s/trinket’s iLvl up a substantial amount. Now, I’m not completely sure by just HOW much, it’s been a while. But I believe it was between 20-30 iLvls.
I disliked almost all of the BFA systems: warfronts, islands, azerite armor.
I only liked conflict and strife essence at the very end.
SL covenants, time gating people into only experiencing 1/4th of the content without penalty.
Professions.
Pet Battles & weekly pet battle bonus.
i have to agree on the fact that it did bomb gathering professions.
but garrisons could haver been something great if blizzard would have work on it properly being honest here.
There are a lot of contenders for this one but I think that Azerite Armor is the winner of that crown. The item level ‘upgrade’ was essentially a player power catfish because the inversely designed Heart of Azeroth system(HoA) acted as a soft reset within the Azerite Armor design.
Quick note: Azerite Armor didn’t have any secondary stats because they where removed to compensate for the Azerite Powers that they were designed around. Also helm, chest, and shoulder pieces were exclusively Azerite Armor slots. Moving on.
Since the Azerite Powers got numerically stronger the higher the item level(lol) they were gated behind a higher HoA level requirement before they could be activated. This meant that players were objectively and literally nerfed by equipping higher ilvl Azerite gear until they grinded up their HoA level that was, to reiterate, a slow experience grind by design.
It was a so obviously flawed and backwards system that Blizzard completely removed it as the expansions borrowed power system in BfA season 3. I honestly think that the HoA and Azerite Armor dev teams never met until it became abysmally obvious that they had to. It was an absurd system that never should have made it off of the early drawing board.
Shadowlands. That “system “. 
Any and all system that causes every one to be the same and kill class / character uniqueness like:
Covenant abilities (everyone getting flesh craft etc)
HoA everyone getting the same abilities
And also PvP gear having ranking system . When you make a new alt and have to grind your way while literally getting 1 shotted is not fun and too time consuming.
Also this alone is causing massive stat inflation going into next expansion. We’re goina be level 70 with over 200k hp now . Good job blizzard item and level squish to literally inflate it back in 1 expac
/rAnt
HoA/essences/Azeroth gear. I just really hated it. Having to have multiple sets of gear for each activity and hoping for the rng to get the right piece out of the vault. Corruptions were kind of weird at first but when they let us farm and choose what ones we wanted that was some crazy fun at the end.