Diablo is like stacking blocks. Stacking blocks is fun. Diablo 1 gave you some mixed blocks to play with and you could make some fun towers. Sometimes you'd get the Butcher, sometimes Skeleton King, and so on. Some of the uniques items you'd find really defined a playthrough.
Diablo 2 gave you more blocks and more stackable shapes. You got skill trees, more clasdes, more item parameters, and just plain stuff to wedge on a more intricate tower.
Diablo 3 and 4 appear to have attempted to bedazzle the existing shapes and allow players to buy stickers. There was little, if any, innovation beyond revenue.
Like, Blizzaed saw Grinding Gear Games acquire money through cosmetics and thought that was what people were over there for. (And they mostly just implemented Final Fantasy 10 sphere grid... and solved the 'gold/currency' problem.) But I have to admit I was so lacking in interest I never bought 3 and 4--only watched someone play a bit is all.