I too have stopped reviewing the 3-star books for the most part, and that's simply because I have so many unread books I really want to read that I can't be bothered to spend the time finishing it if it's just meh.
What I try and do with books I didn't love is to try and explain WHY I didn't love it. There could be a reason that applies to me but might not apply to a reader. For example, when I read exercise video reviews, people will sometimes feel very strongly about the music or the outfits or something. If that's why they marked down a title and it's not something I care about, I can weigh their review accordingly. It's the same with books. If I mark down the book because ti features (for instance) a major focus on the character's dog (there seems to be a sub-genre of mystery where this comes up a lot) and you adore dogs, you might feel differently about the book than I did
If a book is really terrible, I generally don't finish it either (life is too short) but I do write a 'sample review' which is explicitly a review of only the Smashwords sample. In such a review, I explain why I did not get past the sample and into the book itself---why I gave up, in other words