I reviewed books for a science fiction site for a few years, but have since given it up due to time constraints. It was a large community with a bevy of different reviewers from different backgrounds. Everyone who reviews books or films has their own style, and it's pretty open because any review is opinion based. But there's one type of reviewer that always makes me cringe and long to hunt them down and slap them repeatedly.
Now granted, a review is an opinion piece. But that doesn't mean it's all about you, the reviewer. You can present the plot of the book, and your thoughts, positive or negative, without ever having to use the word 'I'. Too many book reviewers take what's supposed to be a commentary on someone else's work, and turn it into an opportunity to stroke their own dick, or simply talk too much about themselves. For instance, go ahead and say a book is reminiscent of 'To Kill a Mockingbird' and why. Do not however add that reading said book reminds you of the time you got that award in your high school lit class for having the best comparative essay on Harper Lee, and your Mom bought you a new phone for being such a good student. No one gives a fuck.
Another one that we see a lot is the dreaded phrase, "As a writer…" These three words should never, ever, ever show up in a book review. Firstly, if you're writing a review, we already know you're a writer. But more importantly, it does not give you more credence as a book reviewer, in fact, it does just the opposite. It's great that you're a writer too, but that you felt the need to add that to your review of someone else's work makes you sound like a tool at best, and at worst, a frustrated, failed writer with insecurity problems. Sure, plenty of fiction writers also review books, and there are ways to reference your own thoughts and preferences, to use your expertise without turning the subject away from the work you're analyzing and putting it back onto you.
It's okay to add a bit of flavor to your review, some humor, a personal touch. But again, this can be done without saying things like, "As I sat in my kitchen, eating a leftover sandwich my wife made me, I wondered again why it's so hard for someone like me to find a book that keeps my attention. Maybe it's because I grew up in New York City, the most exciting place on earth." This is not a human interest piece about your personal life. It's a review of someone else's work. Same goes for comments about what kind of reviewer you are. "People say I'm a bitch" or "I'm really picky when it comes to books", dear God, shut up and write the review, you attention whore.
So if you review books, go ahead and express your opinion, be creative, add a bit of flair if you want. But try to keep the focus on the work you're discussing, and off yourself. If you're a creative writer yourself, show the reader that by using your tools. Not by being a tool.