I told a friend recently the secret to writing a good review is to FOCUS on what you like, MENTION what you don't and not SPOIL the ending. Reviews are important to authors, because no matter what is written, the fervent desire is that it be read. A review is a means of communicating to the author your impressions of their work. While readers benefit from these observations, writers live for them.
A good review does not focus on the negative. Anyone can tell you what they don't like about something. Rather, the whole point is to tell the author and other readers what you like about the work. If you have complaints, they can be noted after you give the author their due.
Readers find it difficult to write reviews and I can understand this. It is hard to put into words your reaction to a story. Perhaps, you think the author already knows what you would say. Or maybe you are afraid of spoiling the ending. While these are all valid concerns, understand that an author will not know what you think unless you tell them.
Having written many highly-praised reviews myself, I feel that I can provide a few tips in the writing of a review. One of the easiest things to mention is your favorite part or aspect of the story. If the ending is your favorite, then remember that spoiling it for others will diminish their enjoyment. Instead of that, try discussing what lead up to your favorite part without actually giving it away.
Another easy thing to mention in a review is your favorite character. Since they are the life's blood of the work, your review will come to life if you focus on them. Discuss their good qualities and even their shortcomings, all while being mindful not to give away the final consequences of their choices.
Everyone wants to be appreciated, and writers are no exception. A well-written review helps an author gauge how well they communicated and how it was received. It can also build enthusiasm for the work among the readership. So, in review, accentuate what you like, communicate what you don't and abstain from deteriorating the enjoyment of others.