Diablo a Devil of a game
The first would be the basic idea of the kill and loot aspect. Through the game you would have to kill creatures and obtain weapons, spell books, and armor from their bodies. The items could either be regular, magical or rare and allow your character to level up and grow to an almost unlimited level.
The second aspect would be the spells, which at times would be hard to find but when you do the spells were devastating. The spells like the character could be leveled up but the spells leveled through reading books instead of experience. The spells could be used by strategically firing them behind a jail cell or around a corner. The most entertaining spell for me was the firewall which you could use to ignite the entire room devastating all creatures in it.
Besides these parts of the game I believe there is one thing that above all places Diablo at the top. This part of the game would be the addition of multiplayer. Most if not all role playing games during this time were comprised of single player alone, which was fun but halted the re-playability of the game. The addition of multiplayer allowed people to form clans and the PVP allowed characters to fight vast battles over the internet.
These three I believe are the main bullet points of the game, but in order you really understand how good this game is you have to play it for yourself.