Being bad at WoW is keyboard turning and clicking your actions.
This is WoW just becoming unnecessarily fiddly.
Everyone played encounters with boss mods on, so they made encounters more complex to match. To the point where something like Ra-den Mythic has so much shit going on every guild just goes fuck it, and makes everyone use the Weak Aura. Even then it just says something like "VOID" and you have to know what that means.
As for M+, it's a case of rote memorisation. The mechanics will kill you on high levels, no way to figure them out as you go along because you'll be dead. Run them on lower levels. Boost a noob group through a +1 - +5. If you can explain the mechanics, you're ready to do them on higher levels. They don't change, it just gets so that you can't make any mistakes. The only thing that really changes is that on +10 and above you can skip enormous stretches of irritating trash.
Everyone who has been playing since the start forgets that not everyone has.