What makes a Diablo/ARPG clone fun?

What makes a Diablo/ARPG clone fun?

While making new levels for my flare mod i keep wondering this, what makes a Diablo/ARPG clone fun?.

Level design? Too little elements and its boring, too many and its visual noise, especially since you are not supposed to be in the same place for long. Small size is harder to create than bigger, too big and it feels empty.

Enemies? A nice variety is good, but actually all of them are the same thing with different aspects(not to mention recicled ones). They are always either fooder, tankers, ranged or magician. You can give them a twist like suicide bombers, static spitter plants or spawners, but even that gets old. And flare has it limitations, so combat is not very dinamic.

Loot, if it changes hero aspect, the better. I myself think this is plenty important, collectable sets or pets catering to completionist. I would even say its half the game's fun. There is a basic crafting system implemented, but i can barely see any benefit between crafting yourself a potion or buying in bulk from a shop. Or farming for weapon/armor materials when it can be just a drop from a mid-boss.

Difficulty, the hardest thing to balance, and it boils down to having enemies that are damage sponges, or can almost one hit you, or fodder with relative danger/annoyance factors. So you need to grind for better loot at an easier map, i guess this is a common thing for arpgs.

Story, if its interesting, the better. But its just there to give a excuse to whack things in the head. So far im going a tiny bit heavy on it.

What are your opinions?