Was one of the original 1000 D2 Beta testers, and then the D2Exp 1000 testers :-) Game has a lot of meaning to me, got so excited when the beta disks came in the mail :-D
D2Beta Disks if you never seen what they look like.
Blizzard often doesn't get the credit it deserves for pushing really good control schemes in their games. Both SC and D2 can be played with a mouse-only mentality, and it's a big reason why their games are so popular.
The skill system, while pissing off some people, worked well for newcomers because it prevented hoarding all of your skill points into a single skill. Along with the item mechanics, skills felt like real rewards - you leveled, and you got something great for it almost every time.
It came at the peak of 2d isometric graphics in games. It's art direction is unbelievable. It is the darkest, most gothic video game made. If you own The Witcher on GOG, you can read through it's digital art book that is a downloadable extra, and they talk about how in their early design meetings Diablo 2 came up as the style they really wanted. Witcher was the first realistically dark game I had seen since D2.
The soundtrack, another often ignored portion, marked a real shift in composing for games. It's almost musical noise, avoiding melodies in most situations. As a result, it's creepy. But it's not just creepy, it lacks musical resolution, which means it keeps the listener expecting something to change. In other words, the music makes you want to keep playing.
The items. Oh dear lord the items. Diablo 2 had the best items ever, because mods were so flexible and disjointed. You could build a character around almost any single skill, or any single item, and it would be hilarious. It wasn't going to be the most efficient, but almost all of them were effective enough. As time went on builds became popular, but playing through the game and just using what you picked up made the game constantly change for you.
Diablo 2 is said to have killed ARPGs for a time. That it was so good, anything that followed was decried as either a cheap imitation or simply not like Diablo 2. Many believed that Diablo 2 was one of the first modern grind-fest RPGs, and as a result many that followed were painfully grindy. But Diablo 2 wasn't designed around the grind; players would grind in Diablo 2 because they didn't want to quit. That's how good it was.