I haven't played Mass Effect, but here's my take on a few more challenges that need to be overcome in order to make the game really appealing.
I think what combat provides in a lot of games is the threat of death and punishment. Avoiding some sort of death and punishment can be a very fun and motivating thing in a game. The adreneline rush from a close call and barely pulling off a maneuver in games is very satisfying.
A game like mario can easily get away with very little combat, because its mostly about avoiding pits and obstacles. Mario really doesn't have combat outside of the few boss fights. I wouldn't really consider goomba smashing combat, its just a jumping puzzle you can squish.
I'm a huge fan of monkey island and grim fandango, but they never provide an adreneline rush b/c theres no skill involved besides thinking.
If a game was based on dialogue paths, what is the fear from choosing certain paths? How will the game punish you? Will certain decisions make the game harder and make you regret making that decision while still being fun? If so how...