Ok here's my opinion about Diablo:
It's a Hack N Slash/Action RPG. You can create your character, you level up, you get to increase a stat or get a new skill(Diablo 2 only). Then you do some quests whilst going through 16 levels of monster mayhem and then you defeat Diablo and game over, start on another difficulty.
An action RPG focuses more on combat then Role-playing elements, that's why you can't "ask for a reward" or "fall in love with an NPC."
If you want to do stuff like what Fifth said about Fallout or Arcanum then go play a CRPG, not an Action-RPG like Diablo.
Definition of an Action-RPG:
http://en.wikipedia.org/wiki/Action_role-playing_game
And CRPGs:
http://en.wikipedia.org/wiki/Computer_rpg