ME1 "fridge horror", mystique and logic coherence.
Example: Feros mission. What happened here:

1. Ruins of an old presumably prothean city are home to the Thorian, an ancient lifeform which has high knowledge of protheans.
2. Saren needs that knowledge, so he came and made a deal.
3. Shepard needs the Thorian's knowledge too, and Saren, being aware of that, sent his geth to kill the Thorian.
4. The Thorian used his enthrallment abilities to protect himself from the geth.
5. Shepard, being oblivious to this situation, came to Feros solely because of the geth presence.

Bonus points from mystique: there is no explicit statement on where the Thorian came from and how the protheans interacted with it.
More bonus points from "fridge horror": being forcibly used to fight off synthetic butcherers is a terrible fate on its own. Good thing the colonists don't know that they were test subjects...
This simple story stems from actual plot (protagonist and antagonist want the same thing); the player doesn't get an infodump explaining everything in detail, instead they explore the location bit by bit and eventually come up with the story themselves.

Later games don't do that, look at Javik, Leviathans, Cerberus and overwhelming number of linear pre-explained pathways to fight through. Not making the player the only driving force of the plot is not what many games do, gotta respect ME1 for being successful at that