What makes Mass Effect awesome (in my book) are the party interactions, the (relative) freedom of character, and the galaxy-shaking consequences.The short version is they wasted an opportunity to make something as awesome as Mass Effect.
Party interactions they could have done. But not so much the freedom of character, nor the galaxy-shaking consequences (or at least, not without stepping on the movies' toes).
Something awesome like Mass Effect would've been nice, but I just don't think it's possible with a stringent license like this, where all the toys need to be back in the pram at the end.
Not necessarily, you could have done much of what was done in Mass Effect with the existing characters. Talk to Sulu, Checov, Uhura, and so on about their respective pasts and give them missions. This is, however, a much more linear game than Mass Effect, though. Linear doesn't necessarily mean bad, though.
I will say, ME is about what I want from Star Trek. It's also, amusingly, pretty close to JJ Abrams movies with better plotting.