What are some the most BS mechanics in the game?

[KageTokage]

That is only because the majority of people don't understand how to see the visual cue to resolve the mechanics.

Puppets bunker 3rd boss, look at the screens. Any white markers on it, need that many more people.
Alexander AOE marker with silver balls floating above it, that's the number.
I know there are more than just those two, but they are the ones that I see most during roulettes.

So simple, yet so hard for people to understand.

"Body count" mechanics are most commonly referring to things that will either wipe you or result in more guaranteed deaths if you don't have your entire team, which can include, but is not limited to stuff like Enumeration.

There's considerably more of them in the present vs. the ARR through SB era and I agree it's rather frustrating when you're essentially required to do every step of a given mechanic correctly to actually see more of a fight because it's impossible to recover if anyone dies at all at specific points.