I haven't read every post in this thread, but honestly don't need to to say what I'm going to.
I started MMO's with Everquest launch. I have played pretty much every major and a ton of smaller ones along the way. I have played unguilded, in guilds, in small "family" type guilds and in super high end raiding guilds that were top of the server and game overall at everything we did. Seen it and done it all in MMO's generally speaking.
With older games (EQ and even early WoW) people, for good or bad, were forced to listen and learn and if you knew how to teach content. The reason was simple, death mattered in the game. If you didn't like 6 hour corpse retrievals you had to think, play your class right and rely on others to do that. With the lack of any real death penalty people don't care (again generally). People run into areas guns blazing and simply don't worry about the outcome because in less then five minutes your back where you just died from without any real issues or negative effects.
This alone has created a MMO community that is much larger then ever before, but is much "worse" then it was also. No need to learn stuff because trial and error works effectively when you can die and come back so easily why actually plan and listen and teach that takes to long for the "need it now" MMO world. Faster for someone to run ahead and die three times trying, and eventually getting the job done, then to teach others a good tactic to use.
"Back in the day" when you joined a group you listened to people if you didn't know and they told you what was coming up and how it would work and if you didn't listen and pay attention you found it hard to get and stay into groups. The community talked, a lot, and you would be done before you even started. Ruthless? Maybe. Harsh? Maybe. That said it created a MMO community that was actually fun. You could join a PuG and do raids, get epic weapon quests completed and such without issues. People were more interested in helping in a PuG because with the smaller community you would run into that guy often as you leveled so you wanted him to know how to play.
Things changed, WoW (can we say that here?) changed things, they started catering to the masses and to be honest made the games much easier. The thing is, that gained them a ton of players so other games followed and if you didn't follow you got run over. MMO's were changed, players changed and it won't go back to how it was. Soloing is the in thing now, games set up classes knowing this. This never use to happen because everyone WANTED to group. Why? Because it was effective and fun. Grouping increased your chances to not die. Now days grouping increases your chances. That's not how MMO's should be.
So the "multiplayer" part (not just PvP) has changed, a lot. Blame the need it now, it's a game why should it "hurt" if I die, this is to hard part of the MMO community for this but the OP is completely correct the "multiplayer" part of MMO's has made some MMO's unplayable and made other MMO's solo fests with only people that get into "guilds" (super groups?) grouping. Again this is general, talking about overall player base, some still trudge along in PuG's everytime they log in and some make it work really well. Overall though there seems to be more people playing solo then grouping something I'd never of expected when I first got into MMO's.