- Currency rewards: the acquisition of a game resource that can be spent represents a fairly universal reward system…
- Rank Rewards: the player gains benefits from acquiring points towards an eventual step up in rank.
- Mechanical Rewards: such as increases in stats that the player can feel the effect of.
- Narrative rewards: a little narrative is effective for certain players as a reward.
- Emotional rewards: when the player feels they have done something for someone in the game.
- New Toys: anything new that can be experimented with is a ‘new toy’.
- New Places: are a mimicry reward for players driven to explore
- Completeness: achieving completeness (chasing 100% for instance) can be a reward in itself.
- Victory: defeating a challenging foe (or a boss).
(This is stolen/paraphrased from the blog Only a Game) via Andrew Chen