BFA was one of the most barebones expansions in the history of the game. The only thing it had going for it was the story, which is a burning carfire. Half of the Horde is acting out of character. Suevanas commits mass genocide on a whim. Morally righteous characters like Dezco or Lor'themar aren't immediately revolting against Suevanas immediately. The Alliance gets to kill Rastakhan but Horde players don't get to kill Jaina as she teleports away and escapes. The Alliance storyline is unsatisfying as they keep getting genocided but are never allowed to actually deal with the problem once and for all. We get sad orc story #47.
Even if you disregard the writing, the questing content isn't very fun to play compared to MoP or even WoD.
Four expansion concepts (war arc, South Seas, Azshara, N'zoth) are thrown away in one expac. N'ylotha wasn't even a zone.
The quality of the voice acting has declined.
The music continues to suck. I have three tracks from BFA on my favorite's playlist but otherwise the soundtrack is a dud. Should have brought Russell Brower and Jeremy Soule back.
Decline in aesthetics. The new character models and armor look really goofy and don't fit the Warcraft universe.
Zones are very mundane and don't feel very fantastical.
I need not mention Azerite power.
No content to do outside of the questing. What, you consider doing your world quest chores or running the same dungeon over and over again on time attack mode to be content?
The one thing I did like about BFA was the island expeditions, which were sadly thrown away and forgotten. I preferred levelling up to cap through them rather than slogging through the questing experience.