For me one of the coolest things about D2 as well as D3 was the nearly endless character customization. Although there were cookie cutter builds, you were never forced to use them. You also weren't forced to use typical items on your characters if you didn't want to. You could rock a sword wielding sorceress and go completely melee, or throw a bow on your paladin to create a totally different character experience.

This freedom helped to spawn a ton of creative ways to play characters. Later runewords were added that gave class specific skills to anyone who equipped the runeword, giving your character even more options. You could have sorceresses that shapeshifted, barbarians using paladin auras, assassins casting teleport, etc.

The other aspect that was always neat was the trading. I loved following the markets and seeing what was worth what. It was like following a stock market where things were constantly increasing/decreasing in value. The idea that you could start to play the game with absolutely nothing and in a few months amass a huge amount of in game wealth was really appealing.

Another thing was the aspect of community with the people you played with. I'm not talking about the typical random bnet users, but playing with friends was great. Although this goes for lots of MMOs, somehow diablo was just different. You could play with a group of people and help each other out not only with beating the game, but also you were constantly finding items that your friends could use on their characters, and vice versa.

You also had pvp as well, so that was another draw for a lot of people to come up with a completely different player combat focused build. Diablo had something for everyone, and it's been nearly 12 years since D2 was released, so I think that's why you're seeing such hype for it.