Well, just like in movies, sometimes what is shown in a demo is not in the final product. Maybe that side quest was pulled, maybe that was something they created just for the trailer... it happens. Recent example: the Iron Man 2 trailers... a certain major scene in a few the trailers is nowhere in the movie.
Personally, I like the game the way it is now. The PC UI was (personally) much better than the trailer's, the dialog seemed forced and slightly awkward to me (idk why), and the asari's voice acting seemed weird. I don't know, something didn't feel right. Originally when I started ME, it caught me off guard how you couldn't directly control your party characters, but that system of control sort of grew on me. The map... I have no idea what to think about that. Part of me wants to see what that could have been in the final product, possibly being a better system, but a bigger part of me likes the system they currently have in place.
IMO - the problems that ME had (bland environments - planets, bland side quest locations - they all occur in a small lab, a habitat, or on a small ship; they all look the same, the vehicle... etc.) weren't covered in the trailer. I prefer it the way it was in ME.
The grass is always greener on the other side. Once you get there, you finally realize that you gave up your prime real estate for a small patch of bland dirt. Idealizations kill desires.