This topic might be sort of dead, but I have some simple reasons why video games are usually tough to tackle in the big screen.
Comics: Already written and established, they literally just take what's there and film it.
Video Games: Choice! This planet or this one? This way or that? Think about it, in Mass Effect you have a ton of choice in where you go, making a movie would be tough.
That's only a tip of the iceberg, my friend. You're forgetting that the main character has no true appearance him- or herself. Not only that, but Shepard has three origins with three possible service records. Shepard's class is also up to the player and even the sex! It's possible that they'll go for the generic default male soldier that's on the box, though. Just saying that Shepard isn't a solid character due the character customization elements in the game.
Putting Shepard aside, there's the various choices you make in the game. Obviously, they'll probably go down the Paragon route, though.