My recollection of Vanilla is that in most situations, if you pulled one group at a time, it was easy. Pull two groups at a time and it's hard. The latter may force you to burn some long cooldowns or lead to people dying, but wasn't necessarily a wipe.
Now, people pull three groups at a time intentionally, and it usually doesn't get anyone killed. Sometimes they pull three groups at a time, one of which has a boss, even. It means the healer has less time to heal before people die, but so long as he can play whack-a-mole effectively, no one dies. Unlike in Vanilla, now everyone has infinite mana, at least apart from arcane mages, so a lengthy battle doesn't lead to people dying after the healer runs out of mana.
In some games, it's beneficial to pull a lot of mobs at once so you can AoE them down quickly. But WoW has little AoE, and so pulling multiple groups simultaneously doesn't even make it go faster. If anything, it makes it go slower because people have to stop to heal a lot more in combat, and time spent healing is time spent not dealing damage. In some cases, it also means that mobs heal each other a lot more, which also slows things down.
And it still seems like most groups pull multiple groups of mobs at once intentionally. It's not just accidents, either; I've seen tanks go intentionally pull three groups successively that were spread out considerably, to the extent that it would have been easier to pull one group at a time. And if the tank doesn't go off and pull several groups at once, someone else probably will.
You might think it's just because I'm low level. But I'm skeptical of that. The other day, I saw someone in Orgrimmar complaining about how awful the random people in PUGs were for mythic dungeons. His complaint wasn't that the groups were unable to finish. It wasn't even that they wiped occasionally. No, it's just assumed that if you throw random people who don't know what they're doing into a mythic dungeon, of course they'll complete it and get their loot.
Rather, his complaint was that they didn't rush through it to beat some timer, and it's unfair that they still got their loot for having cleared the dungeon. Perhaps he didn't realize that that's the point of WoW: you get an epic, and you get an epic, and you get an epic, and everyone gets an epic. It's all about time spent playing; actually knowing how to play is only marginally relevant.
In Vanilla, it was at least possible to make given content arbitrarily easy or difficult by attempting it at a higher or lower level. This had the enormous drawback that nearly all of the game's content was the wrong level for you. On paper, level scaling should fix it. The problem is that the level scaling scales things such that everything is way too easy for you.