I'd say:

1) Seeing the others having fun (including the DM).
2) Finding solutions to problems in front of us. [As long as it doesn't require to anticipate. So more "reactive problem solving" than "preemptive problem solving"]. This include tactical combat.
3) Seeing a narrative unfold. The narrative does not need to be prepared / railroaded, but I like to see how the universe evolve and react to what is happening.
4) Helping at game design for houserules/homebrews, or design of the universe.