I'll repost my comment I had made on a similar article as it's very relevant.
I have been playing WoW since day 1. I've seen the huge cultural shift that, in my opinion, is the real cause of its decline.
The two big things that fell away are player humility and actual progression.
Back in Vanilla, most players were in greens. Most were still learning how to play. There was a small percentage of the player base that was hardcore and everyone looked up to them because it was obvious who they were. Epics were nearly impossible to get on your own. Hell, blues were out of reach for a large section of the population but that was important! It made most players humble. I'd hear things like: Man, look at that item, I could never get that! I'd have to join a top guild and quit my job and put lots of hours into raiding and socializing. People would genuinely FEAR bosses! We'd talk about certain bosses with reverence to their difficulty. We'd feel like kings for a day when we finally took down that boss we were stuck on. That feeling is gone from the game now. The last 3 expansions have lowered the bar to excellence so far that even the most mediocre, casual player feels like they should be able to topple the last boss. Nobody on my server even killed C'thun until the next expansion came out.
Blizzard's biggest mistake came when they decided that all players should get to see all content. They freaked when they looked at their metrics and saw that less than 2% of guilds were doing Sunwell but that made the dungeon actually epic. Could they not see that what made the game so magical and mysterious was knowing that some content was out of reach? I remember going to school and talking to friends about how world first guilds just downed such a boss. I remember the awe, feeling like these people were celebrities to me and my friends. I remember getting attuned to Naxx40 just so I could go in with a friend and LOOK at the instance! I remember feeling so accomplished for getting attuned to instances and working my way up the progression ladder. I remember it took me and my guild a YEAR to get ready to run Kara! We would run content we could do and backed off the hard stuff until we knew we were ready. We knew where we were in relation to every other guild because it took months of prep to move up a tier! You knew that someone had killed X boss because they were wearing Y armor. Progression mattered. There was no jumping up to where the rest of your guild was just because they were running harder content. Accomplishments were palpable. You wore your boss kills. You guild name was your reputation. All of that has been washed away so that anyone can feel epic.
Now... I could roll a character and within 3 weeks be maxed out. This wasn't impossible before it just required a LOT of friends. This need for others to speed up powering your character has been largely removed. You fact that you can be 90% as strong as a raider by yourself cheapened the experience for everyone but the kinds of people who ragequit and call people faggots. Those people, in Vanilla, would get nowhere because your characters strength and prestige was directly related to how social you were.
The catalyst for all this was opening the floodgates by opening cross server PvE. Suddenly you didn't need your social skills to run 5 man content. You need only push the "find me 4 friends" button. ALL of my friends I made in the game were first met in 5 mans. Socializing worked along side progression. You start by yourself, levelling and learning the game, only socializing when you saw fit. You then decided to take the game more seriously and pull 5 people together and maybe make a friend. You'd join up with one of their guilds because they like the way you play or strategize. You'd decide to start raiding and you go to the easier 10 man content. Your guild grows and suddenly you have enough players to go after the harder 25 or even 40 man content! It was a social progression as much as it was a progression through the content. All of that has been washed away in the name of "let everyone see everything!". I'm willing to bet nearly all players who started playing with me are no longer playing because the core engagement of the game has changed so much.
When everyone is epic, nobody is.