- The story, man. The story is amazing. I can't imagine how many hours - nay, days! - it must have taken to craft such an epic yet personal adventure.
- The character and ship design. As rich and deep as the story is, it wouldn't work if you didn't feel immersed in the universe of Mass Effect. The ships are realistic, as are the characters and locations. Every scene is a work of art.
- The variety of missions. Every new "quest" sends you somewhere new and exotic. When you return to the Citadel, for instance, there are new tasks and new faces.
- The battle system. It's easy to learn, hard to master. Lots of different ways to disable or kill your foes. Very fun, almost infinitely customizable.
- The vehicle. Is it just me or is that truck thingy damn hard to drive?
- The important dialogs don't show up in your journal. Sometimes I don't get all the details in one discussion, and I (foolishly) assumed that key points would just be in the journal. But they aren't. Someone says "Go to planet X and find person Y at building Z." Then the journal says "Go to planet X." Ugh! Don't make me buy the guidebook, Bioware!