I think I've figured it out, what went wrong in WoW. It was the arbitrary sorting mechanisms. The arbitrary barriers and filters. No no no, not that there were too many. There were too few!
Imagine that you're in a random group. You have never met these people. What can you expect from them? Skill and persistence are things that you cannot measure at all, just by inspecting them, unless maybe they're in whatever the latest and greatest hardest gear is at the time, in which case you can expect a boring, trivialized run. Unless they're a DPS and you're the tank, in which case you can spend the entire time feeling inadequate at your inability to hold aggro. Unless you're playing post-Viagra patch in which tanks can always keep up their aggro, regardless of experience or interest. Ahem.
How can we sort out these people a bit? Simple: Make them run through some hoops. Make it take time, for no apparent reason, to get to the instance. In other words, remove teleports. Now if you're in a group you can at least know that the people you're with care enough to run over. The exact amount of time is up for debate, but for a start, I think we can mostly agree that going from Ironforge to Zul'Farrak was a bit ridiculous, so less than that.
But maybe this isn't enough. Add more arbitrary problems. For one, remove the "tank, healer, three DPS" auto-formation system. Instead, put players in a giant pool and let them sort it out. If somehow they end up with a shaman or warlock tank (I've been both) and stick around, then they are either so ignorant that they see no problem or they can handle a bit of challenge in their day. In my experience, groups that did not instantly disband under circumstances like this tended to be the most fun. They were not particularly efficient in terms of gathering gear, but if the ultimate goal is fun, why not cut out the middle man?
While we're at it, bring back attunements. Not crazy attunements like Black Temple, which were less of a burden on an individual as on an entire guild/raid. But Karazhan, that was a good one. Perhaps a bit long for the very first raid (the attunement, not the raid).
Is this elitist? Hardly. It's kindness. Let's all think of who we want in our groups, pretending that friends don't count, because that's cheating. Do you want someone who has acted before this very moment to prepare for the activity, or do you want someone who randomly got the idea to join a group, not really caring where? Consider who is going to drop group the moment anything goes wrong. The first person is more invested, not merely by time, but by motivation. They chose to be there.
It's also about managing expectations and counter-intuitively, saving time. The player who leaves at the first sign of trouble will have had some sort of expectations, which are then crushed when they wipe on the first boss. He will have wasted his time as well. If he were instead to leave right after group formation, then he would save his time, and that of the group.
There is a glaring flaw, of course: by-the-book groups will still be formed most of the time and these will do no filtering. People may queue for instances right next door to their questing, and will not be filtered. In other words, this does not catch all the impatient people. And there is the question of where they go. Even if I don't want to group with them, I don't particularly want them to quit either, so they must find groups, assuming they want to. But the only groups that are suitable are the well-formed, smoothly-running groups.
Those would be guild runs. Organized groups. 'Bad' players should be in guilds while good players should be in the chaos of the PUG. But this is the opposite of what happens. Guilds try to keep out the bad players while good players avoid PUGs. Good and bad aren't really the terms I want here. Maybe adventurous and patient vs. conservative would be better. But those don't quite work either. Well, the point remains, the players best-suited to handling the chaos of randomly-formed groups are the least likely to be in them.
I suspect I am the exception, in that even when I was in guilds, I tended to run with PUGs. Part of it was that the LFG tool made it easier to just click a button than ask people. But part of it was that I liked seeing what I'd find. PUGs were how I found guilds and guild members.
To conclude: This entire post is useless, because the sorting mechanisms of arbitrary barriers to content will be overshadowed by the social sorting mechanisms.
But they should still bring back the attunements for Onyxia.