Looking through the reviews on sites such as Amazon or IMDB it is clear that most fall into either clear likes or clear dislikes. This is pretty much as expected as unless someone is a committed revier they are really only going to post their views if they are at one end of the spectrum or the other.
Everybody likes positive reviews and these are all usually fairly similar. Negative reviews can be something of a problem area for the reader though. And the problem is usually with either what the reviewer didn't like or the way they have expressed that.
There are several categories of negative review and I feel some are significantly more valid than another. Some examples are:
Service problems translate into poor product reviews
This is especially true on a site like Amazon where someone will post a 1 star 'review' that simply moans that Amazon delivered the goods late, damaged or in some other way gave poor service. Unfortunately the review rating goes against the author (for books) or manufacturer (for products) and doesn't actually impact Amazon one jot. For small authors/manufacturers who rely on word of mouth and good reviews to sell, this can be extremely damaging. Sure, mention that the service was poor but don't let that affect the rating if it is outside the control of the producer of the product.
I tried it once and didn't like it
You buy (for example) a book, just on impulse, not due to any other recommendations or previous knowledge. It wasn't what you were expecting. This shouldn't really be a negative review of the book.. The review should still be fair and should explain what was wrong. I see far too many reviews that are essentially one-liners such as "It wasn't what I was expecting" and then one star. That's not helpful to another person reading that to tell what was wrong.
If I was in charge of giving names to things (which fortunately for everyone else I'm not) I would call this the "Come Dine With Me Syndrome". On Come Dine With Me (if you've been living in the wilds for years and are not aware of it) complete strangers are put together to throw dinner parties for each other. What happens every week is that there will be a prawn cocktail starter and one of the guests will say they don't like fish and so give the host a lower mark. Why? It's not the host that has been at fault, they should be marked on everything else and the fact that they served something that wasn't to an individual's taste should be ignored.
It was recommended/heavily marketed but I didn't like it
This is excellent territory for a negative review. If you pick up a book (again) but this time it comes highly recommended or with a very enticing marketing campaign and you didn't like it, you are entitled to say that. However it's not good enough to say "it was rubbish" it needs some context. What didn't you like? What did you like? This isn't the same as a constructive criticism but it does give the reader of the review an idea of if they might like it.
Using my superb naming skills, this would be called the "Barry Norman Effect". I used to watch the Film porgrammes with Barry Norman. He would review the major films out at the time. Some he would like, others he wouldn't. But he always said why he didn't like it. And I didn't always agree with Mr Norman's choice of likes and dislikes. But because he was clear what he found good and bad I was able to judge if I would like it.
Yes I wrote a negative review
What brought this blog entry about was that, after nearly a year of writing reviews of every book I had read, I finally got to one I didn't like. I have to admit I was getting a little worried that I liked everything because I knew I was going to review it and post that review in the public domain and didn't want to offend the author... but I finally had a book I couldn't finish and really was probably one of the worst books I have ever had the misfortune to try to read. Yes I really did find it that bad and struggled to find anything I liked about it. You may disagree with me, but then that means that you may be able to judge the next book I write a review for a little better.
The book? A Game Of Thrones. The review can be found here https://www.goodreads.com/review/show/1045309105