To write a fair, objective and thorough review, I recommend you include as many of the following as you feel appropriate:
1. Your familiarity with the author’s work. Have your read other books by this author? Are you a fan of this author’s work? Is this the first of hers you’ve read? This tells the readers what you know of this author’s work and gives your review credibility.
The easiest way to get book reviews is to write reviews for others -- reviews of the books you love.
Should you write snarky reviews?
No. For many reasons. The primary reason is: why did you read a book you don't love? The world's full of books. Life's too short to waste on horrid books. And yes, you should read all the books you review, right to the end. How else can you write a review?
Here's another reason: payback. Think about it... ;-)