progression rates and values of enjoyment in RPG games (loot systems, hard bosses, easy bosses, etc)

I've been programming an MMORPG with my team and we've come to a disagreement with what the player base might find enjoyable with content. Is the lore the best part for most? Do some just play the game for loot-drop excitement and call it quits? What are the core psychological values YOU personally love to experience while playing an RPG? Ideas? Progression systems?