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?
In the 2000s Nick Yee conducted very extensive research on MMORPGs. His Daedalus project surveyed more than 40k players. They tell very interesting stories about motivations to play, for a starter consider a taxonomy of player motivations. Yee identifies three categories of motivations: Achievement, Social and Immersion. Within each of these he identifies further narrower clusters of motivations. He also demonstrates that motivations are not mutually exclusive in the sense that players are usually motivated by a mix of factors.
As for the ME part, if we adopt Yee's terminology, I consider myself driven mostly by Immersion (in particular Discovery and Customization), but also by Teamwork and Mechanics, maybe Competition.
When I apply this taxonomy to my games and gamified websites, I try to cater for each of the motivation clusters. For example, incremental points allow for a range of achievement driven motivations. For some bosses you could also give perks - this could cater for some discovery-driven motivational drives. The possibility to chat (negotiating the loot?), or various forms of teams or competitions would enable social interaction as a motivating factor.