What's so hard about this?

We raid sometimes.
When we raid, we have to fight bosses. Bosses are really big things with nasty teeth 9 times out of 10.
That's not the problem.
We run with 3 healers and 2 tanks. These healers and tanks often have to scramble to be able to keep the fight from degrading into a bloodbath made of Dead Warlock.
That's not the problem.
The problem, you see, is that we have some people in the raid, who somehow. SOMEHOW. No matter how many times we do a fight, make the same mistake.
People who somehow don't think about what their abilities do when they use them.
If you have one tank for Adds, and one tank for the big boss, and some adds show up and go over to the boss tank, you do not use your "taunt 3 things to you" ability, mister add tank.

Our add tank did that. I hated him more than anything in the world. Not only did he let me have adds for a good 40 seconds during the fight, but when he taunted, he used an incredibly risky move that ended up getting him then all of us killed.
did that really HAPPEN!?
At the point in which you raid tank, you have to understand communication with your raid. you have to TELL them when you're going to do something like that. If he had said "using righteous defense" I could have taunted the boss back! Boom!
But no, he just silently used it, the boss went running over, and I just stood there bewildered for a good 15s wondering why I suddenly didn't have threat.

Stupid people aren't worth bringing.
It doesn't really matter how casual your guild is. Raids aren't casual. If you go into a raid and you say "eh, maybe we'll get a boss kill" that's very different than saying "Lets get these guys down and get loot!" The second is our Raid Leader's position, and that puts us very out of the casual box.

once you are out of the casual box, even by a little bit, you have to say "no, I cannot bring these people to raid, because the other 8 have the understanding that we will be getting this done right."

You just can't! bringing one or two people who have no clue what they're doing means the other 8 suffer. It's incredibly frustrating. It makes me want to say "I dont go unless X isn't tanking", which makes me sound like a dick.

Maybe I am a dick. But I can do my job.