The big issue with WoW is, that it never bothered to train it's playerbase for what it threw at them. If you quest right now, you will never be challenged. It's all very streamlined and a breeze. You will not get knocked down, have to get back up and challenge it again head-on with a different tactic. The only reason a lot of the Classic playerbase, as well as a good chunk of the TBC community experience WoW as "too easy" is that this factor has been removed. Before there was a lot of ways to fail at the game, this has been massively reduced. Obviously there are other factors as well, like new enchant systems, gems, talent revamps, class revamps (Paladin Pre-2.0 anyone?) and how the gear scales each expansion.
But the major factor is: If you do not teach a player the game, they will not be able to handle what is thrown at them. The community took this over from Blizzard with Addons and Guides years ago. And now the old players (or those that informed themselves a lot) feel the game is too easy, the new players still think it is too hard and Blizzard has to find a impossible middle ground due to their failure to do the most basic thing of any game ever made.