I have no idea what the low rating people were expecting. It obviously is not an Oscar movie and doesn't want to be. It also did not want to have some grand plot. It's just a nicely done action and gore move in a post-apocalyptic setting.
Also, for the review complaining about lack of chemistry between the leads and the wife's unrealistic transformation: the wife clearly said they were getting through a divorce when "it" happened, and that she had been heart broken. Her character at the beginning of the movie is clearly depressed and not facing the realities of their world, she looks half "asleep" and afraid all the time. I found her change in personality to be quite natural for someone who was awakened by the brutalities they experienced, who realized she wanted to live and that she still loved her husband a lot and was prepared to act to defend that, and her awakening was completed when the last pretense was shattered at the end, the hope that her parents might still be alive.