Minus the principal actors, many shots were 100% CGI. Sometimes the principal actors were also CGI. Again, 964 video gamers.
I'm surprised, given your obvious wide knowledge of film, that you really think having a different director wouldn't have made that much of a difference to the final product.
The overall structure of the story and the net resultant outcome would have remained the same, no matter who was sitting in the chair. Whedon is credited with the "tone" of the script (primarily referring to the yuks), which may have been different with someone else. The content of the story and the visual style were pre-ordained and out of the director's control.
Compare and contrast Exorcist: The Beginning
(dir. Renny Harlin) with Dominion: Prequel to the Exorcist
(dir. Paul Schrader) if you want to see an actual example of such a thing happening.
As for the "flat script", it would appear you're in the minority in that opinion.
Wait until the paid reviewers move on to the next hot thing. You'll remember how much you paid for the tickets longer than you'll remember the movie.
Almost every unpaid review I've read of this thing points up uncanny similarities to Transformers 3.