Diablo II was my second experience with the elevated third-person view (the first being X-COM). I borrowed it from my sister's friend one summer, and kept it for two months. I never did finish the game; just kept grinding away with my aggressive-spell-casting Paladin. Dial-up online co-op was new to me, so I didn't spend much time online - just spent hours and hours pointing and clicking. Good times.
When I look back to Diablo II, several things really stick out to me. The first Diablo had only three classes, and apart from looks and stats, they were very similar to each other. Not so in Diablo II, where each class was totally distinct and played quite differently. I played through the story once with each class, but Sorceress and Paladin were my favorites.
Another key improvement over the original was in the variety of locations. A parched desert, deadly jungle, and even bizzarre astral levels really made you feel as if you were exploring an entire world, not just a dungeon beneath one town.
Some of my favorite co-op moments came fairly recently. When Diablo 3 was announced, I got together with some of my old buddies and we played through the game yet again. The game has aged extremely well: partially because of the patches Blizzard faithfully rolled out often, but mostly because it was so well-crafted in the first place.
What's not to like about Diablo 2, really? There's something for everyone to enjoy, from the casual clicker to the hardcore binge-gamer. If Diablo 3 is as good as Diablo 2, or perhaps even better, I shudder to think what will happen. Gamers everywhere may lose their jobs from calling in sick days. Household chores will go undone. I suggest we just go ahead and declare Diablo 3's release a national holiday; who is with me?