For the most part, each of the specs is getting way more of a unique feel compared to past expansions. The big issue for a long time was homogeny, as each spec and class got closer and closer to feeling the same. What they decided in Legion was to go to opposite route, making each spec feel very unique with a heavy "class fantasy", like how Combat rogues are now called Outlaw rogues, and have a very pirate-esque feel. Shadow Priests now have a very Old God focused slant.
I think this is mostly possible because of the drastic changes to the PVP system. As far as PVP gear goes, it no longer works like normal gear outside of PVE, instead each class being based on a stat template. They can add to their utility with honor levels by getting unique PVP-only talents, but for the most part Blizzard no longer has to worry that "this class and this gear is too powerful due to this skill" since they just calculate the template based on what they want to class to do. The only way to increase ones power is to increase ones item level, which will in turn increase by a % the effectiveness of the template. A PVP outlaw rogue with 690 ilevel will have exactly the same stat makeup as any other PVP outlaw rogue at 690 ilevel.
Raiding itself is also becoming less "you have to be this class or die" and feels more inclusive these days, which gives further room for the specs to be more unique without Blizzard feeling they are causing everyone to skip over certain class/specs.
