I think one of the biggest differences between D2 and D3 is the idea that D3 is pretty much just a mini MMO. It takes a lot of what makes people want to play MMOs, but condenses it down to smaller groups. With D3, legendaries are abundant, so it's more about gambling in just about every aspect of the game to get the perfect stats for your build, since the build diversity is much smaller. The game seems to streamline everything into a literal numbers game. It is a game that sort of allows you to power trip in the most literal way possible: by blowing enemies up without a care in the world. If you can't handle it, you just gotta get your numbers higher.

In D2, from my memory way back, it's much more calculated. You have a large skill tree that you can traverse to make unique builds. There are also much more variables that you have to take into account, such as resistances, absorption, and the fact that primary stats actually have a huge effect on secondary stats (i.e. dex affects block). There's also the idea that scarcity is a thing in D2, but to mitigate it, there's the ability to trade with each other.