I have yet to give a 5*review to any book on Smashwords. Like any cataloging system the 5* rating system popularized by recent editions of Windows, is reductive. There are a variety of criteria by which a book can be judged but in my experience the majority of people use only one - the plotline. They don't review the book, they rate the plot which leads to an unrealistically high rating much of the time.
The problem is deeper within society. Some years ago when I was looking at different marking systems around the world I saw the effects of grade inflation on academic work. To offer a hideous generalization, it is not that American academic work is superior to that in Europe but a top grade in Europe can be obtained from a much lower percentage than is customary in the U.S. Americans have narrowed the grading system and there is a general expectation that the average student will achieve a higher grade than the average in Europe. When people grow used to a narrow banding system where anything below 90% feels like abject failure they are going to generally rate higher in other areas of life.
Even when you try to apply a range of criteria the system is imperfect. Looking at the ratings on my own Calibre library am I really saying that Cory Doctorow is the literary equivalent of Charles Dickens? Stylistically they are worlds apart however the ending of Old Curiosity Shop, much praised in its time, does nothing for me, whereas that of For the Win brings a tear to my eye. Taking all the differences into account, they rate roughly equivalently in terms of my emotional engagement and pleasure which the books brought to me overall.
People generally have unrealistic expectations. My own work,
The Gods of the Terminal, is a slight thing. It started as a short story and became a novella. I wouldn't put it out there if I thought it had no merits. There are some observations that anyone who has ever done telephone tech support may recognize and some satire of the modern financial sector. However in publishing it I do not believe that I am hugely expanding the literary canon in some way. Most people think that they will be the next J K Rowlings though. For that reason they expect all 5* ratings. If you have a wide group of friends, and access to something like Facebook, it isn't too difficult to gain a flood of such ratings if you really want to.
This isn't something unique to Smashwords, any of the book or technology selling sites are potential victims to abuse of their rating system, including reviews which are posted by employees of the publisher/producer and are designed only to raise the book in the overall rankings.
If you look at Smashwords reviews the majority of people write little more than a line, yet it is the review not the rating which tells the full story. I try to never write less than a paragraph, sometimes several, and when I read the reviews I look for people who have taken into account things like dialog, characterization, imagery and weigh their opinions higher than those who just say how much they enjoyed the story.
I agree, it is not easy to write a negative review. We humans are fragile things and our egos are easily hurt. I wrote a review the other week that took me half a day because I wanted to be balanced and reflect the strengths and weaknesses of the book and because it had received nothing but praise from all the other reviewers. Within an hour the author had unpublished the book. I felt extremely guilty. All literary opinions are subjective and this book which many others had enjoyed was now unavailable to anyone. I like to think the author is working on correcting the myriad of typographical and formatting errors which I had highlighted and he will in time republish it but I don't know for sure. Perhaps he was just angry and upset.
I don't get any pleasure from saying negative things about a text, quite the opposite. I do think I have a duty to be honest and ego-flattering, whilst desirable, doesn't help anyone learn or write a better book.