I've heard complaints that it's 'too space opera' and the main species are 'too human' - in appearance and culture - to have a major, thought-provoking impact in science fiction
Right, because Vulcans, Klingons, Bajorans, Betazoids, Romulans, etc. look nothing like humans and their cultures have no basis in any human cultures in history. That's possibly one of the dumbest arguments I've ever heard.
The Mass Effect universe is an incredibly rich one ripe for development. There's plenty of potential for excellent stories beyond Commander Shepard's. The books are a good step in that direction, but what it really needs to "become the next Star Trek" is to move into wider mediums. Sadly, video games are not that medium right now. Far too many people still think of them as "children's toys." A film or television show set in the Mass Effect universe (done well, of course) is probably what could bring it into the mainstream. In fact, the best bet would be to not even include Shepard or any of the game characters. Keep them to the games. Use the other media as an opportunity to expand the universe, and build the brand.