If we look at an endgame tier list for diablo 4 class builds we find in the S-tier & A-tier (or most efficient builds) use most of the same skills. I believe this creates a big problem with diversity that I never personally felt playing Diablo 3, at least not to this degree. While some classes have good diversity, half of them resort to using near identical abilities for “high level”/“efficient” builds. I enjoy the systems of D4 place in favor of a Set of armor that pushes 1 exact build along with the power creep. The systems (Aspects and having many affix’s on gear) allow for making adjustments to builds that are healthy, such as choosing tankiness, damage or utility over one another. I hope in future updates skill diversity in builds will be greater in “high level” play.
Barbarian (Whirlwind, HotA, Rend) - 4 skills : 3 shouts and Wrath of the Berserker
Sorceress (Arc Lash, Ice shard, Firewall) - 4 skills : Ice armor, Flame armor, Frost Nova, teleport
Rogue - 1 or 2 skills (Much Better 
)
Druid - 1 or 2 skills (Again much better )
Necromancer (Bone Spirit & Bone Spear ; even Sever) - 3 Skills : Corpse Explosion, Corpse Tendrils, Blood Mist
This was pulled from the Maxroll website tier list and their builds.
I believe that this is a somewhat accurate look at the diversity of the best builds as of now.
Loved Diablo 3 played a bit of 2 and cant wait to see the future of Diablo 4