So many people, myself included, buy and read a book then move on without another thought. Whether you enjoyed the book or not, leaving a review is so helpful, not only to other potential readers but to the author as well. Reviews don't have to be long or professionally edited ; >) Anything you liked or didn't like about the book will be beneficial to the next person who's considering reading it.
It could possibly even help the author improve his or her writing. If there are things that you really enjoyed or didn't enjoy about the book, leaving a quick review noting a couple of things, good and/or bad, might draw the author's attention to something they hadn't noticed before. Perhaps even leading to changes that make the book or future writings even better.
Not much can be done about personal taste. If you don't like the style of writing or if there is too much or too little of things you are or aren't looking for (sex, drugs, violence), it's okay to make mention of that in your review, too. It might help someone else decide if the book is something they want to read. I think it's only fair to mention in the review that the book was well written (if it was), just not something you enjoyed.
I think the main thing to remember is that every book is written by a human being. Please be kind in your reviews! You don't have to sugar coat anything but you don't have to tear the work to shreds either. ; >) A simple "The book had too many typos for my taste." as opposed to "The book must have been written by a five year because no adult could write that bad." would be good. The first statement can be improved upon. The second, well unless the book was written by a five year old, that's just degrading the author without adding any useful input that could help improve the work.
If you usually don't, start leaving a review for every book you buy. "Great plot." "Great characters." "Slow start." "Lots of typos." If you feel like leaving something longer and more detailed, all the better! If you already do leave reviews, that's great! I'm sure every author you've left a review for appreciated it! (Unless it's the five year old comment from above. ;>)