I don't understand all the fuss about Hollywood making the Movie. This is one of the Games that deserves to become a movie and Hollywood, even though it has produced a lot of crap, has also produced masterpieces. Frankly, it's not even Hollywood's fault that some Directors and/or producers/screenwriters are s***heads who can't count beyond 3, let alone make a nice movie. It's not Hollywood's fault that it gives the chance to anyone with a low budget and an idea, the ability to shoot a film. Sometimes this is a good thing rather a bad one (the story of Spielberg's beggining comes to mind).
That said, the fact that the creators of the game will take part in the process of the movie-making speaks for itself. I don't think that they will kill their own franchise, with a crappy movie. This will sorely hurt their future income as a company, so my guess is that they will try and stay true to the flavor of the game and the characters in general.
As for the movie itself. Considering the typical films of the genre my guesses are that:
1) Shepard will be male. Don't have something against females heroines (Sarah Connor ftw!) but males are dominant in action films for the sheer fact that Males are the dominant sex in such areas as war/battle/conflict/physical antagonism which is an integral part of Mass Effect etc.
2) Taking the above as a given, I will surmise that a relationship will be introduced with Ash. Again, don't have something against Aliens

, but an antropo-centric movie (Human is the main hero and one of the main plots in the game) clearly defines an anthropo-centric point of view so that the viewers could better envision themselves in the hero's place.
3) A lot of money will be given for SFX and make-up artists. As I stated above, since the creators of the game will be part of the team and seeing as how BioWare's games are so polished, I can only come to the conclusion that they will work as diligently in the movie, or even more so, providing guidelines, correcting mistakes in the scenario etc.
4) Mass Effect will be the new Epic Sci-Fi Trilogy which hasn't be produced since the days of the first 3 Star Wars Films (Episodes IV-VI). I have every confidence in BioWare's Team and in Hollywood's decisiveness to spend exorbitant amounts of cash to make a good movie.