A friend of mine (thanks Debbie's Spurts!!) posted this thoughtful question, and I felt called to provide my answer.
I write two types of reviews: regular, full-length reviews and mini reviews that I call "Bullet Reviews". While almost any book could get either treatment (and often I'll write a "bullet reivew" and then proceed to a full-length review), I do have a few rules differentiating what type of book receives what.
Most non-fiction and graphic novels along with books outside my regular genre of scifi/fantasy/urban fantasy get the "Bullet Review" treatment. It's a snapshot of what I thought about the book, no more than a couple of paragraphs at most, but I don't feel that I can give a true opinion, as I haven't really experienced a lot of other books in the same genre. This criteria is especially true with graphic novels - I am very much a newbie to the genre and outside of maybe Star Wars graphic novels, would not be a very good person to ask about continuity, art, flow, writing, etc.
Most other books however will get a full-length review. These include:
+ DNF's (I think it's VERY important for me to indicate WHY I had to stop reading)
+ Book Club books (unless I didn't even crack them open, that is!)
+ Amazon Vine books (duh, it's a requirement)
Once I've determined whether the book will receive a long or short review, it's time for me to embark on the review itself. I tend to write reviews in this pattern:
+ Plot Summary
+ How I came to read the book and initial thoughts
+ Writing Style
+ Negatives or positives I couldn't address earlier or ones I want to emphasize
+ Summary and final thoughts - also recommendations
If the book is particularly bad (such as with the Earth's Children and Anita Blake), I may write a snarky plot summary and THEN use the final paragraphs to wrap up final thoughts. I don't typically write meta-reviews (like this brilliant review), because I'm not nearly clever enough for that.
I review and rate most every book I read - good, bad, ugly. My 5 star reviews tend to be exceedingly boring. It's mostly "ZOMG, this is awesome, I 1000% recommend!" OK, maybe not THAT bad, but I've found it's much easier to pinpoint what is wrong about a book than to gush about what was right.
My rating rubric is below:
1 star: I hate it. Serious flaws in writing, character, and plot. Do not recommend.
2 star: I didn't like it. Flaws in writing, character, and plot, with some good parts. "Not for me" books and DNFs may be here. May or may not recommend, depending on what the flaws were.
3 stars: It was okay. The flaws equal the good parts or the book may just not be my genre. DNFs may be here as well. Typically recommended with caveats.
4 stars: I liked it. With the exception of a few flaws, I liked this book. Recommended.
5 stars: I love it! Why has it taken me so long to find this? Highly recommended!
And that's really it! I go through waves where I am on fire for reviewing, and then suddenly I don't want to review anything I read. But even then, I still try to squeeze out a review, because I write them for the following reasons:
+ The potential reader
+ Myself - to figure out what I feel about a book
+ My friends, who have put up with endless snarky status updates and want a payoff :)
My reviews aren't for the author; an author might find one of my reviews very well written and that's awesome. Or maybe an author loves the criticisms and input I have into his or her story. That's great too. Or maybe an author just ignores all reviews he or she gets. That's perfectly fine as well. But I don't write reviews for them.
On average, it takes me about two weeks to finish a ~300 page book. That's two weeks of time I could have spent doing a lot of other things - knitting, crocheting, baking, playing Munckin, watching Game of Thrones, or reading a better book. Not to mention, I shelled out however much money for the book. I get that not every book will be my thing (and for that, I almost NEVER rate less than 3 stars), but I do expect quality work. My time and money are precious to me; I hate for a book to squander that.
This is why reviewing is so important to me. The reviews I come across help me decide what books are for me and what books are not. I've found AMAZING books reading the reviews of my friends. And these reviews weren't just "ZOMG, this is AWESOME, the guy is so HAUT!!" No, they were detailed, thought-out reviews that indicated that A) the reviewer took a LOT OF TIME to write them and B) really wanted to share their love for the book.