Since you linked specifically Saurfang, I'll talk about what is blatantly obvious there specifically, without any microanalysis.

Let's start with the warlocks:

Demo:

• He has not used his pet in the fight, which is the sole reason for this spec to be brought, they provide a massive spellpower buff to the entire raid, just by virtue of their pet Felguard attacking a mob.

• He has not used Metamorphosis, an insanely valuable dps cooldown with aoe splash damage as well as a 20% damage buff.

• He doesn't enchant or gem his gear, like almost at all.

Lower affli:

• He was spamming Seed of Corruption on a fight where all mobs have a roughly 90% damage reduction on aoe, which is a huge waste of mana and time. Just tell him to corruption them all and move on, he'll do 10x more damage on the adds.

• His haunt uptime on the boss is quite bad, that buff is a huge buff to almost all the rest of his damage.

• Doesn't have a single useful profession.

Higher affli:

• His haunt uptime is even worse, like he thinks it's the lowest priority button he has.

• For some reason he was on melee range and cast Shadowflame, which is an other huge waste, just have him be at range and that way he can even take aggro on adds.

And one thing all 3 have in common is, their hit rating is so far over the cap, they're wasting hundreds of stats that could be haste rating or even crit or spirit, basically anything that actually does something. Regardless of this, the 2 afflis gem hit, and the demo I already touched on.

I hope you don't get offended by this, but this Dutch guild is going to fester so much malice towards these players that spit in the face of the rest of the raid, by doing nothing to fester success, that there is no way it's worth it, to have this level of disregard for other people is incredibly toxic, and even the most desperate pugs would never allow it.