While Mass Effect may not have "atmospere" as you call it (and you're right, it is VERY repititious), the game really hooked me in with how in-depth everything is. Especially the Codex and the world blurbs. It amazed me that each world had it's own little biographical blurb giving it's size, gravity, atmosphereic pressure (if any), orbital period, and even distance from the primary. And the Codex, jeez. How many other games (or movies, or books, or TV shows) that take the time to explain to you how heat is managed on a ship? Or how where you choose to fight in a ship dictates how long you can fight for? Or pointing out that the ships in this universe need to discharge an electrical build-up, either by landing and grounding directly to the planet or by hovering in a planet's magnetic field and dumping the charge into that?
Little touches like that, which have NO bearing on the game whatsoever, was enough to make me overlook the recycled level design or those stupid Mako missions.