The news that Lois Tilton has resigned from reviewing for Locus reminds me to mention my personal theory about constructive criticism:
There's no such thing.
Criticism is criticism - it's always some kind of fault-finding, and it's always personal, because some person did or created what you are criticizing.
The fact it's so common doesn't mean its helpful or even useful.
I've always felt criticism should be an internal process; rather than just lazily blurt out the problem you see, analyze the problem, and then offer a solution.
That's called advice.
The criticism is implicit in the advice, but when you're at least trying to be helpful, the subject can skate past the fault-finding and go straight to improvement.
Many people also criticize others for things which are really matters of personal taste. If you go on a date, and you order chocolate ice cream for dessert, and your date asks for strawberry, you don't jump up and shout:
"You're a complete idiot! How can you like strawberry ice cream! What a sorry little asshole you are!"
Of course, those of you who follow science fiction these days know most criticism sounds like this. That kind of reaction is the product of privilege, from people with a very limited world view who have grown up never hearing anyone disagree with them because of their political, social or financial privilege.
Last year most of the criticism of Hugo nominated works was done in bad faith from people who simply deep down don't like the fact the authors existed at all. The fact they weren't aware or couldn't acknowledge their preconceived biases is immaterial.
Most of the reviews of my short story "On a Spiritual Plain" boiled down to "The premise sucks, and it's a weak story, and it's badly written, and Lou Antonelli is a miserable human being, anyhow."
Occasionally I was surprised by some genuinely thoughtful reviews. Any author worth his salt will recognize VALID criticisms. For example, saying a story of mine relies too much on dialogue and first person narration is valid; I lean on that a lot, and it indicates a weakness in my writing skills.
But IMHO, overall most so-called constructive criticism I hear simply reminds me (having been raised a Catholic) of original sin. Deep down, we're all sinners, and it's something we all have to fight constantly - to do good and help people, and improve the world.
Constructive criticism is usually just a justification for hatefulness.