Mass Effect does have a shitloat of dialogue, but I imagine that if you streamlined it you could cut the story elements down fairly well. It's not a complicated or overly intricate plot line compared to many many games.
What GPW said.
My point is it's hard to take a game that is played in say 30-50 hours and try and make that work in a 2 hour movie. Hell, it's been hard for them to get 10 hour games and turn them into a film!!!
There is a reason why most 10 hour games struggle in film form, and it is because they have very little story telling, and most of it has to be added by the screen writers.
Mass Effect's game's world is so rich with many wondrous sites, and great character and back story that any good screen writer should be able to put together a fine story.
You will probably see an ensemble cast featuring Shepard, Tali, Wrex, Ashley, Kaiden, Dr.Liara, and Garrus... though they will probably go deep with only three or four of those... like say, the new Star Trek flick.
Beo's idea about covering a separate aspect of the ME universe is interesting, and probably a sensible one, but I don't think it will happen, as it won't be financially sound to ignore the recognizable characters. They want to sell this movie with Shepard's face on it.
Wow, I feel like playing ME again. That music you hear on the Normandy is in my head.
Any ideas who they should cast as Shepard?