Diablo is mostly killing stuffs, the deal is this.
Every character has 3 skill trees, every skill tree has like 12 skills on it. You can learn and level up a skill with a skill point, which is granted every time you level up, this mean you have 99 skill points (there are 11 more skill points for quests), this make impossible to have all skills on level 20 (max level a skill can have without a item that gave +X point on skill or skills). So building a character is extremely hard, because it has to be good for the game (pass Hell Difficulty that has immune monster with absurds amount of hell and you have penalties on your resistances and for pvp). This make 2 things: 1.- leveling up a character is like a science, and 2.- there are million of buildings.
Example. Barbarians are the mele character of the game, but you can make it range if you want with different builds and skills. The Amazon is the range character of the game (with bow and javalines), but you can give her a sword with a different build (I think this build is named Angel or something like that).