Diablo II took a game about descending one dungeon and turned it into a genre: skill trees, sockets, runes, item rarities, and mercenaries were all bolted onto the original's click-to-kill loop and became permanent fixtures of the action RPG.
Follows: Diablo
Diablo's characters were defined by their equipment and their class; you did not so much design them as accumulate them. Diablo II's skill trees changed the relationship. Points spent were points committed, and with three trees per class and a finite pool of levels, every choice foreclosed others. The result was that players began to talk about their characters in terms no Diablo player had needed: not "my Sorceress" but "my Blizzard Sorceress" or "my Hammerdin," identities built from a chain of decisions that could not be casually undone.
This is the deeper reason the sequel outlived its predecessor by decades. A game where the character is a set of accumulated items is a game you finish. A game where the character is a hypothesis you are testing — will this combination of skills, runes, and gear work? — is a game you restart, and Diablo II was engineered for restarting. Brevik's shower idea, borrowed from a strategy game's tech tree, turned out to be the mechanism that made an action game infinitely replayable.
Where Diablo's items were rewards, Diablo II's items were a parallel system with its own rules. Quality tiers established a vocabulary of rarity; sockets turned a good item into a platform; gems and runes turned that platform into a puzzle; and rune words — precise sequences of runes that unlocked defined properties — gave the most dedicated players a research project that ran for years. The economy this generated was real enough that trading became a genuine metagame, with player-established currencies and value hierarchies that Blizzard never designed.
The consequence was an endgame that had nothing to do with the plot. Diablo dies at the end of the story; players kept playing anyway, running the same acts thousands of times on higher difficulties in pursuit of items whose drop rates made them near-mythical. That structure — the story as an on-ramp, the loot chase as the actual game — was the sequel's most widely copied contribution. Every loot-driven action game since, from Torchlight to Path of Exile to Diablo's own sequels, is working within the architecture Diablo II established in 2000.