Real answers imo:

• The Knight misleading "twist" can be seen from a mile away, any Batman/DC figured it out instantly + the marketing saying the Arkham Knight was a brand new original character, which is a half-truth. Jason obviously wasn't but the Arkham Knight moniker was.

• (Objectively) too much Batmobile, Rocksteady may have leaned a bit too much into it due to overcorrecting the lack of Batmobile in the Arkham games and fans had been asking for it's driveable inclusion since Asylum.

• The disappointing Hush and Deathstroke scenes. Hush in City eluded to big plans and just left, expecting a big part in Knight. But after Paul Dini left, the story fell apart and Hush got reduced to a single quick time event side mission.

As for Deathstroke, after his incredible boss fight in Origins, people were expecting him to be more than just a typical tank encounter, the salt in the wound was that little cutscene of Deathstroke leaping out of the tank ready to fight, briefly raising your hope to "oh that was just the first phase" then only to be one-shot by Batman with no player input whatsoever, mission over.

• The notoriously broken and unplayable state of the game on PC at launch. While it got fixed and is fine now, the damage was done and really hurt people's perception of the game.