Highschool of the Dead's concept is what makes it so acclaimed. It's trite but not trite at the same time. While the zombie apocalypse stuff was (and still kinda is in some areas of the media) hot shit, there wasn't anime that portrayed it in the way that people would expect (if there were any that portrayed it at all for that matter). It's like people would scratch their heads and wonder "why the hell hasn't there been an anime with a zombie apocalypse in it?" before HOTD came.
So, despite your conception of the plot's quality it deserves credit for being an anime that made it with that particular concept.
Not that you have to have it in your top 10, of course.