Couple of things.
1. Diablo 3 is one giant cliche, the story is balls.
2. I already played D2, same thing.
3. It isn't really random at all, aside from the crap loot system and the dungeon that may or may not be open sometimes.
Your opinion is that the game is great, and I respect that and hope you are happy playing it. I however hope that blizz maybe puts a little more thought into the story and gameplay of future titles, rather than constantly going straight for the totally expected plot twist and the "age of mortals" endings.