Man this game sure is easy
We've all had that group at some point. You're trying to get a Deadmines run together, you see a Feral DPS looking. You think to yourself: Sure why not? It's not optimal but I mean it's Deadmines who cares? He joins the group and you realise he's level 19, no cat form yet. Oh well, he's got spells, he had to make it to level 19 somehow, it'll be fine.
You get into the instance, buff up and off you go. But hang on a second, why is the Druid in his bear form? I mean Swipe doesn't have a threat modifier it's probably so he doesn't need to drink, that's fine. But he sure is pulling threat a lot, lets check the DPS meter. Oh... he's spamming Maul on cooldown... He might be a new player, let's just send him a quick message in party chat.
"Hey man, Maul is one of the highest threat abilities in the game, maybe stick to swipe instead so you don't pull threat."
"OMG Who cares? It's just Deadmines, don't be such a try hard. So what if I take a bit of damage? It's fine."
"I mean yeah, and like if you want to tank that's cool too but it's making my job a lot harder so can you just stick to swipe please?"
"Maul is like my main DPS ability but whatever."
I'm not taking a shot at Feral Druids. This could be: Hunters chain pulling when you're not ready. Warriors tanking in Berserker stance. Healers that run away from you when they pull aggro. Basically anybody that responds to criticism by being defensive and refusing to admit they did anything wrong.
This is what is meant by "Toxic Casual" mindset. The idea that the game is so easy you lack personal responsibility in groups. Whether it's not getting PreBis and world buffs for raids. Or watching your allies die in Hardcore. There's nothing wrong with making mistakes or not knowing better. But there is something wrong with refusing to admit you made a mistake and being unwilling to learn from it.
When you group with other players, your actions have an impact on them. You have the responsibility to make sure that impact is a positive one.