This is a special one to me! Still have to figure out what to aim for in such a game.

Rarity

Items
   Specific Items/Class Items
   Enchanted/Corrupted
NPC's
   Quests
   Empathy/Influence/Guilds?

Diversity

Skills
   Weapon animations
   Item related Skills  Skill modifier on Items

Consequential Decisions
Equipment
Team Composition
visuals/layout

just a short write down...
comments are wellcome

By "item related skills" do you mean skills which become available to you when you equip a certain item? I am a fan of this idea, but I am wary it can become OP. Maybe if you limit this feature to sets of items?

you mean because of skill overload? maybe I should call it skill modifier on Items...

The idea is diablo3 related - legendary may remove a skills mana cost or hit 3 times in a row...

Someone recently was talking about skills on items (found it) and I'm not sure if this will work any way, was theory crafting only, haven't tried it jet.
Though in theory I was thinking about skill diversity through items (mainly on weapons) as a classless thing but that's a little outdated. If this still find its way into my mods the "setbons" idea might be a good thing to go - so sets get more important compared to unique/legendary items