The world of book reviewing is changing/has changed. And it's changed faster in the USA than it has in Australia. When my YA novel, Dying to Tell Me , came out in the US, I was actually surprised at how many bloggers had been given review copies, and how much influence they had, let alone the number of reviews that gradually appeared on GoodReads. It very quickly dawned on me that our system in Australia (which basically consists of a review in Magpies, a review (much later) in Reading Time, and the very occasional review, if you're lucky, in the news media outlets) is slower and generally still very traditional.
Yes, librarians everywhere do still look at reviews, but more and more, with time and money restraints, they're relying on recommendations and requests from library patrons (including kids), awards lists and general "noticeability". Or the current buzzword among publishers - discoverability. Reviews have moved down the list of important places for your book to be seen and talked about. But not entirely.
Still, with the internet the way it is now, and publicists at publishing companies keeping a close eye on who is blogging reviews and how much notice is being taken, it's worth thinking about where you sit, if reviewing is something you take reasonably seriously.
Professional - these are people who are paid to write reviews. Or who write reviews for prestigious outlets simply for the kudos. These are also the avenues that many would consider "traditional". How does an average review go? There's a fair amount of space devoted to summarising the plot (or scope of the book) and associated elements. The rule of thumb is that you don't give away the big moments and twists, or the ending. A lot of reviews of this kind are pretty bland, although some reviewers like to gain a name by being as critical as possible. Michiko Kakutani of the New York Times would probably be one of these.
General via Amazon, GoodReads - there are people who are famous for reviewing on Amazon. Their reviews are taken seriously, as are their recommendations. They take their reviewing seriously, too. Some people on GoodReads do the same. But GoodReads tends to be more democratic, I think, and more relaxed. You will get opinions, praise and criticism, but not couched in academic language. On the other hand, Amazon has been in the news for "sock puppet" reviews, where people sometimes review under an alias for scurrilous reasons (such as slamming books by their rivals - who'd have thought?).
Your own blog - there are lots of readers now who have their own review blogs. I'm one of them (occasionally) when a book strikes me in a way that I want to talk about. The difference with me is that I tend to approach a review from a writer's perspective. What did I learn about writing from the book, good or bad? What aspect of the book showed me something new, writing-wise? I think blogs that review books rise or fall based on several things - the reviewer's perspective and approach, whether they bring something new to the discussion, and whether they are truly interested in talking about books. They gain followers in the same way I look at the film reviews in my newspaper - some reviewers I don't even bother with because I know their tastes are nothing like mine, others I will read and take notice of.
Word of mouth - yes, this is the one we have no control over but everyone wants. It doesn't matter if you're a publisher, an editor, a book publicist or an author. You hope that readers out there will go around saying to their friends, "You've got to read this book - it's amazing/really good/will make you cry/ keep you up at night, etc." Really, all you can do as the author is write the best book you can and cross your fingers.
But ultimately, in this day and age, word of mouth has the most power at the moment. Flogging your book with a million tweets and FB posts won't do it. Readers are quick to feel put off by this. A super-duper website won't do it. Readers who love what you write will do it for you. So it comes back to the same thing we always talk about - you have to write what's in your heart,w hat you're passionate about. And if you're a part-time book reviewer? Maybe consider that the way to "pass it forward" is to only talk about and review books you love.