Here are some things I like (or don't like) in a mystery book. I am NOT a fan of gratuitous violence and fortunately many of these books have little of that. I wouldn't read them if they did. Yes a murder is usually always committed and it's not pretty, but it doesn't have to be described in gory detail. In my opinion there is enough of that on the daily news.
Since I love to laugh, I like a touch of humor. Some series, notably the Stephanie Plum books, have many Laugh-Out-Loud moments. Others are more serious. As long as they are true to the character, I can enjoy both types.
And a little romance never hurts to spice things up. In my opinion it keeps the story from getting too wrapped up in the main plot. It's an enjoyable sub-plot and keeps me turning the pages.