Honestly, I feel like, conceptually, BFA was quite good. They put a lot of effort in it and that's easily proven (we had 4 full patches instead of 2, see WOD).
The huge problem though are the insane levels of grind and time-gating which served no purpose apart from filling Blizzard's pockets:
- dailies giving 50/75 rep
- honor level grind with no actual rewards (it takes 4 times the honor than it did in legion to reach the max prestige level)
- visions time-gated to 3/4 per week
- island expeditions are incredibly boring (apart from pvp ones, I'll give them that) and their sole purpose is to have the players grind them for the mounts
- corruption is even worse than tf when it comes to rewarding you for grinding them out
Now, it had other issues, but those were present in other expansions as well:
- insane level of stat inflation (we have like 8-10x more dps/health than we did at the beginning)
- weird class balance (seriously, every single class has like 5 heals, 5 absorbs now...)
- troubled story