They destroyed the fun of the trading economy by making the auction house the most efficient way to obtain things - even allowing real-money purchases for a while to make it literally pay-to-win until they walked that back.
They entirely removed the concept of "builds" as everything about your character was changeable at whim, so you just had to max out a class and you had access to every build within that class. Made choosing builds feel unimportant.
Then, they didn't add enough interesting unique item effects, so item grinding largely just became "look for the item with a slightly bigger number"
There's something about character progression that seems very shallow. The overall feel and experience of the game is closer to an arcade action game like Gauntlet (3d version) that has repetitive levels as oppose to an ARPG with RNG content that has endless replay value like D2.