I've been lurking in this thread for days, and tonight my regular group had multiple people missing, so I decided to experiment and go pug instead and see how much of a toxic cesspool it really is. I'm normally a tank, but don't really want to pug as that with these affixes.
So I started out as a ilvl 221 (showing up as 224) boomy with something like 950 r.io score in that spec. I think it won't be too hard to get into groups since treants are really good on necrotic, and people apparently don't look at the shield symbol next to my shiny tank r.io score. Got invited to one of the first four Mists +15 groups I applied to. People say hi. We quickly get a healer and then a tank. I make a joke about making really sure we have enough NF seeing as we are 3 druids, hunter, warlock. Nobody responds. Someone else asks if we're using lock portal to skip. Yep. Key starts, everything goes fine, we have 4 deaths by the end of the dungeon 3 of which are dying to standing in bad stuff. Nobody says a word about it, key is timed after all, even if it's just +1. People say gg ty and leave. I dunno man, this ain't the toxic cesspool I was promised.
Let's try again, now my r.io is 960, surely this will help. I apply to SoA, almost instantly get into a group. We eventually get a tank, people say hi. Key starts, some deaths on the first big pull, nobody says anything. We proceed to first boss where tank gets slapped and dies, combat ressed, dies again. We reset the encounter, healer says "why ?", tank responds "normal hits". We have another wipe, healer tells says "dude dont turn back to the boss", a dps chimes in with "bro you getting beamed". Tank just says "no" and pulls again, dies again. Key owner says "dont need to continue, it's already gg". Group disbands. Not a nice experience really, but I wouldn't call it toxic either.
Okay, another try. Get invited and kicked from two different SoA 15 groups with the leader saying something like "cya". Why? I dunno. Maybe they actually looked at my dps r.io.
Get into a third group. People say hi, key starts, there's some communication about things like rogue shroud and soaking on third boss. We have four deaths without a comment. We time the key +2. Totally carried by the rogue who did 800 more overall dps than me and the other guy. People say things like well done and scuffed but wp.
Maybe I need to get into a more poisonous environment to find the toxicity. This just won't do. But my r.io is 1002 now! Plaguefall it is. I get into the second PF +15 group I apply for. Well, it turns out it's actually for spires. Been there, done that, I apologize for not wanting to do spires and leave.
Get invited to the next PF +15 group I apply to. People say hi. Key starts. Someone pulls an extra plagueroc in the very first pull and tank asks who did it and then requests that we die or the route is fucked. Finally a bad word! But is it toxic? Doubtful. We reset the plagueroc and continue on, first boss is the smoothest kill I've ever had. Tank specifically requests "dont pull bird" as we get to the second plagueroc. Well, nobody does. Everything is fine until after the third boss when the healer dc's after jumping down the hole. Tank suggest clearing trash slowly without the healer. We do, with me and the elemental shaman offhealing when needed. Healer comes back, we spawn pride, then go kill the boss. Two people die to plague crash, one of them being me, so I apologize. Nobody comments on it, key was timed. People say gg ty and leave. My r.io has now improved to 1073.
Obviously a very small sample size, but I'm starting to doubt the claims of rampant toxicity in the pugging world.
Also just for fun: the r.io scores of the people in my three completed runs:
Mists 15: Tank 563, Healer 1092, Hunter 1581, Warlock 1316, Me 950
Spires 15: Tank 1187, Healer 1436, DH 1088, Rogue 2056, Me 960
Plaguefall 15: Tank 924, Healer 1298, Monk 1174, Shaman 1310, Me 1002
TL;DR: Pugging +15s doesn't seem very toxic to me, even with some mistakes, wipes and complete failure involved.