I've already talked in generalities about good and bad criticism, so let's look at the trademarks of each type.
Makes Vague Comments
I didn't like it. It was boring. I thought it was stupid. I hate it. It wasn't my thing. Etc.
This is a sign of someone who hasn't done their homework. Either they didn't read the piece or they read it, set it aside and gave it no additional thought. This is disrespectful to the writer and to the work.
Skips Positive Feedback
Some people may be fans of "harsh truths" and feel that there's no reason to buffet the more negative comments others may simply not think about doing so.
But people need encouragement. It makes us feel good and makes us more inclined to accept and apply criticism.
Confuses You with Them
"I don't agree with this/that." "I like this better than that." "You should change this because, in my experience...."
Questioning the facts is one thing. Thinking a story should change based on your personal likes/dislikes/worldview is another.
If the critic has a view they want to express, they're free to do so in their own story. They don't get to do it in yours.
Makes It Personal
"You have no talent." "You can't write dialogue." "You're not funny/original/etc."
The epitome of bad criticism.
Maybe you do lack the skills to be a "great" writer, but that doesn't mean you can't be a passable writer or even a good writer. But you need help to get there and this kind of criticism is no help. (Well....unless you're one of those people who exists to prove others wrong.)
Takes time with the work and is constructive
Good critics read a work, do their best to understand it and provide a meaningful critique that will better the story.
Meaningful critique being, of course, identifying specific elements that can be changed to make the story better; for example, suggesting a different point-of-view or the expansions of a scene.
There's almost always something good to say about a piece.
And the good critic will find it. It could be as simple as liking a character's name, a piece of dialogue, a description.
If you truly can't find anything, you could simply say "I'd like to see it again after you work on it." You leave the writer with a bit of encouragement.
Good critics don't assume they know where you're going with your work. They don't assume their experience is greater than yours. They don't assume their knowledge wins out.
If they don't know why you've done something, they make a note and they ask.
Basically, the good critic does the exact opposite of everything the bad critic does. They follow what could be considered a golden rule of criticism: give the feedback you want to receive. Their feedback will help you write a better story. Because that's what they want to do.
Up next, some methods (chocolate? deep rhythmic breathing?) on dealing with criticism when it comes your way.