There are also Seasons with unique rewards to obtain, but you have to start fresh each season. You can choose to go through the Campaign mode (recommended for first time players), or even do Adventure mode for the loot grind. Each character has a different play style, so they're all unique in their own ways. In Diablo 3: Eternal Collection, you can pick from seven different hero classes: Barbarian, Monk, Demon Hunter, Sorcerer, Witch Doctor, Crusader, and Necromancer (the last two are from the DLC, which is included with Eternal Collection).

I personally have put hundreds of hours into Diablo 3 across multiple platforms (Mac, PS4, and Nintendo Switch) since the game originally came out because I absolutely love it that much. Don't want to hop on the WoW bandwagon or just don't want to end up with another subscription fee? Then you should try Diablo 3 instead.