It's not just addons. It's the fact that you can find detailed strategies for boss fights, class guides that tell you exactly what to do on everything from reforging and gemming to spec and rotation, what professions to pick, etc, all over the Internet. A person who has never played WoW before and really wanted could be given a random level 90 character and learn how to play it on a decent level in less than an hour by Googling all that stuff. That kind of stuff was around but wasn't as readily available back in some of the older expansions.
These "the game is too easy" complaints have been going on since Vanilla though. Everquest players back then were complaining that the game catered to casual players, it would never amount to anything, it wasn't a "real" mmo, etc. The game was meant to be that way. It would never have gotten the amount of subscriptions that it has reached over the years if everything were heroic only and required 40+ hours a week of playing to make any form of progress.