So far I've mostly only been writing reviews for 2- and 3- star books, which I suspect is because I'm annoyed at having spent money on them, usually through being mislead by the 5-star reviews.
Part of this thought is derived from reading this Zen Pencils quote. It's really really easy to be hard on obviously-starting-out people who are apparently just throwing what they have up on the Kindle store and hoping for the best because man are there a lot of places to criticise. But if they want keep going and get better, then that benefits me down the line.
...but on the other hand, I feel like I ought to have context-invariant standards for categories of things that I pay for.