Bit late to the comments but heres my take. I played WoW for a few years since vanilla (not very early vanilla mind, but maybe a couple of months before TBC) and back then it was hard to level. The quests were not streamlined like they are now.
Back then it took me a long time to level, mainly because I didnt know what I was doing as a noob, but also because there was no big yellow arrow pointing where to go, where to find such-and-such, where the next quest is and so forth. People had to Look for what they were after. Look for the next quest and so on.
Now, WoW is so easy to level and play because it holds your hand the whole time whilst questing. You get to top level in next to no time due to fully streamlined quests that you never have to go hunting for the next place to go. That for me is whats killed it.
I would estimate when I first started WoW and got past the "I have no idea what im doing" phase, then it would take me a few month to get to max level. Now, its about a week or 2 for a casual to level to max. Heck on my first character I stuck with (Warlock) it took about 8 months to get to level 60 - probably because I sucked, but also because I liked exploring and finding stuff for recipes without buying them off the AH. That doesnt exist anymore, its all about BUY ALL OF THE THINGS!!! and ACCEPT ALL THE QUESTS WITHOUT READING!!! and that isnt a game, that isnt fun and its why in my eyes WoW is dying.