Most people who've talked about Highschool of the Dead have mentioned two of the main things that I love about it: Violence and fanservice.
However, for me, the main appeal to this series is how the characters are treat and how they come across. Unlike the countless other examples of every other zombie show/film, or even any horror movie, the characters are always either incompetent, unrealistic, or Main Characters. See, in HotD, the characters are realistic, and make realistic decisions. In a recent chapter of the manga, one character began to break down due to the stress of the decisions and harsh reality of living after the world has ended. So, for me, the realism of the characters is what makes this show great.