Since everyone's been doing it, I might just make a post on the subject myself.
I review every book I read. It might take a while, but eventually all my read books will get at least a short review besides their star rating. Not all these reviews are favourable, but they are always honest. I don't normally give a whole description of the story, usually just a couple of sentences, unless the blurb given really spoils the story far too much for my likings, or there are several plot points that need to be discussed in more detail.
Details that really get my ranting in reviews are a) things that just completely don't make sense in any way except 'convenient plot device' and b) biomedical bullshit. Genetics, for example, is a wonderful tool but don't abuse it like they did in Allegiant. Don't get me started on the ridiculousness of an anti-radiation vaccine. I know it's Science FICTION, but in these cases I cheer for humanity to be destroyed.
Since GoodReads still doesn't allow for half star rating, over there I just make do with what I'm given, rounding books with half stars either up or down depending on a mixture of factors including the position of the moon, the weather, the time of the month and whether or not I just had chocolate. Basically I just use gut feeling to decide.
My rating scale might be a bit different from others'. For me, most books fall into the 2.5 to 3.5 stars category, like a beautiful normal deviation. It doesn't mean I didn't enjoy them. I'd even say that there are books I gave 2 stars but still enjoyed. For me, a 3.5 stars rating means I really enjoyed a book, to get 4 stars or more, there needs to be something that makes it either extra special or memorable.
DNF (Did Not Finish) - Something I kind of never do.
0.5 stars - Avoid like plague. This is reserved for the worst of the worst books I wouldn't dare curse anyone with. It's a shame they got published. Should probably have DNFed.
1 stars - The book was bad, I didn't enjoy it at all.
1.5 stars - The flaws of this book were too severe to enjoy the book.
Recommended (more or less or for certain people):
2 stars - Either the story really wasn't special (in which case it might be a nice snackbook), or just not for me (in which case other people might still really like it).
2.5 stars - Enjoyable, but there were some problems, so I don't think everyone would like it.
3 stars - Enjoyable, with less flaws or a more interesting/original plot/characters.
3.5 stars - A good read. It might not be the most original/memorable book but I really enjoyed myself.
4 stars - I really enjoyed it, it had something extra that separates it for me from other books.
4.5 stars - WOW! I want another book like this. Right now!
5 stars - Reserved for my all time favourites.
Small note: This is how I rate my books now. I used to be more generous when awarding stars, but don't feel like rereading everything to adjust the ratings.