WoW requires a $15-a-month sub to play. This can definitely deter some people from playing. There is a trial mode, but it is very restrictive.

WoW doesn't offer an engaging story [until endgame] or anything beyond point-and-click gameplay, especially with the many other MMORPGs following its model, which leads it to quickly become tedious.

Vanilla Wow used to have a very slow leveling process, this allowed people to experience and enjoy the world and dungeons as they leveled up. Unfortunately, this is no longer the case and the leveling process has been completely destroyed by being made trivial. It is so fast that if you enter a dungeon you will almost certainly have out leveled any loot you find by the time you finish it. Not that most people worry about loot while leveling as most players have access to heirlooms which further trivialise the leveling process and turn your character into a leveling God.

In the early days of WoW there were significant differences between the classes; classes had unique skills which defined that class and giving it its identity. Unfortunately, over the years, most of that variety has been stripped away and now all of the classes have minimal differences other than being ranged, melee, healer or tank.

While this was done to (unsuccessfully) bring balance to the classes, it ultimately resulted in a lot of the flavour of the game being stripped away leaving a much blander experience overall.