D1 had a great variety of enemy subtypes. They may not have been distinct gameplay-wise, but the visual distinction between enemy subtypes added a lot of variety to the game. The different appearance of Hive, Fallen and Vex and Cabal helped to give each world its own distinct identity and would go a long way to bring immersion and reduce the sameyness that the different worlds and enemies have right now in D2. Right now all the enemies look the same no matter where you go, in D1 the enemies on each world are a different subset of the enemy type which went a long way to make each world feel alive.