Making a film in the ME universe is a fine idea you could limit the content to make it easy to digest and not interfer with the plot of the games , however making a film based around a sandbox style adventure, with a prominent moral choce system and a huge amount of content and lore to cover in the main story alone seems unlikely to work. What would annoy me in particular is to see which choices become considered 'cannon' in respect to the film, we already know that the film considers male shep to be the protagonist (which alone I'm sure many are dissapointed by).
What gets shep considered for spectre application? The skylian blitz, akuze or torfan? (If they bother to give him a back story)
Who becomes the main love interest? (Which I'd like to say 'if they choose to have a romantic subplot' but we've all seen enough films to know it will happern)
Who dies on virmire? (Including potentially wrex)
Do shep's action reflect a paragon or renegade motif? (rachni queen, saving the council etc)
Ultimately you end up paying to see effectively a more cinematic version of some-one elses playthrough of ME1, though condensed since there is no way they can fit all the content into the film, and ultimately something memorable in the main story to will be cut or changed.
Personally I would like to know how you are going to give character and/or backstory to each companion in the time available to anywhere near the extent that was within the game. For me personally it was often some of the small things that won me over for certain characters (eg Garrus and tracking down Dr. Saeleon) that I'm almost certain would get cut for time for a film.
Would it have been so hard to make a film based somewhere else in the ME universe? Why not the first contact war for example, it has only being briefly described in the game so a story can easily fit within it and since it's mankinds first exposure to and discover of the greater galaxy it would be a nice way of pacing out explaining the lore (the audience learns as the humans do sort of thing) allowing time for what is probably the most important thing in something bioware the development of the characters.