We all know that Blizzard moved Metamorphosis out of the Warlock class in order to allow the introduction of the Demon Hunter class pre-legion, but one class alteration tends to fall under the radar; The change in the DPS and Tanking spec of the Monk class.
In MoP and WoD, the Brewmaster spec had the distinction of being the tank that could dual wield. This included the ability to wield warglaives and other 1h weapons. This was changed in Legion, where the Demon Hunter became the DW tank, and Brewmasters were forced to only be able to wield staves. While that was more fitting of the Brewmaster lore, the Monk class lost a very distinct feature that set their tanking spec apart from other tanking specs, and the Monk class lost a little flair as well.
The Windwalker spec was also altered in Legion. While still being able to equip 1h weapons, their weapon-based attack animations were removed so that regardless of the weapon equipped, the Monk would only attack with fists. Again, while more fitting of the class lore, it felt incredibly cheap, disappointing, and just plain weird to get a beautiful pair of butterfly swords that remained on your back while you attacked.
Additionally Monks lost a few other abilities and distinctions with the arrival of Demon Hunters; In MoP and WoD Monks were regarded as the "mobile class" thanks to their abilities Roll and Flying Serpent Kick. However, Demon Hunters took that mantle with their dashing abilities and double jumps. Demon Hunters got abilities very similar to Guard and Gift of the Ox, which further eroded the distinctiveness of the existing Monk class.
The point? This is why it is important to push for truly distinct classes in WoW. Potential classes which are too similar to existing classes will only cannibalize existing classes and make them worse off than what they were before. Hopefully, the inclusion of the Demon Hunter is the last time we see Blizzard bring in a class with such a limited amount of design space that took abilities and concepts away from multiple existing classes.