For your final point today, I have a counterpoint:
Improving the medium is only part of your role - it covers your role as a critic of the medium, but not as a reviewer. You may do this for yourself, but we "laypeople" often read/watch critics' reviews for a different reason: To figure out whether or not we want to watch a movie!
This is the same issue I have with reviewers bringing personal beliefs into their reviews and (for instance) bashing The Passion of the Christ for being "too Christian," while praising "Agora" for being secular (and rather extremely so!) Sure, you can argue that your intended audience is the group of people that agrees with you, and if you're one critic out of two hundred providing an alternate viewpoint from the mainstream, this works - but when you're in the majority of critics this catering to a particular viewpoint hurts everyone involved: The filmmaker, for creating a movie that didn't agree with the critics' tastes; the consumer, who may see a "3/10" average score - derived mainly from worldview disagreements - and decide that the film is a poor-quality work overall; and the critic, whose credibility will be damaged for those who actually go see the film.
So anyway, where were we? Oh, right - improving the medium. Perhaps you think it's OK to use viewers as your bludgeon to nudge a samey industry into new territory, convincing them that a movie is bad because it's derivative or unoriginal and thereby sinking the movie's box office; but that is very dangerous ground to tread. The other part of your job, and the whole reason that reviews exist, is to tell us whether or not the film was GOOD. And, frankly, docking points for originality or worldview just doesn't help. Make separate scores ("You'll enjoy it if..." entries?) if you want, but lowering a film's overall rating damages our interaction with the film, and damages our trust of you. Because we don't always view films (or games, or anything else) as an art form. Sometimes we're just looking for something to consume, something that's good, regardless of whether we've seen all the plot twists before. And that is something an art-focused "critic" reviewer cannot help with.