Giving them -
Prose, I've learned by being a judge here a lot depends on characterization and plot - interesting characters doing interesting things. But of course what is interesting to one person may not be to another, so any comments would be from my perspective. This may very well be useless to the writer, they need to decide if I'm helpful.
Poetry, I have a hard time with. I've mentioned before it's like trying to read the poet's mind. Often the poem looks like random words from the dictionary tossed on to the page/screen. I rarely comment on those as my sarcasm meter starts blinking. When I do comment, though, I do try to be helpful.
Courtesy and respect are most important - thank them for their comments and if you have a question ask it. Remember they are taking time and effort to give advice. Take it with humility and professionalism. One valuable thing I learned (other than having thick skin) is that if you feel you have to defend your work something is probably wrong with it. If a person attacks you, ignore it. Nothing will be accomplished be striking back.
My favorite example is the time someone complained my protagonist didn't learn from his mistakes. Well, it's a character flaw - but the more I thought about it I realized it was one of my own character flaws. I was putting too much "me" into the character. So it was useful after all.
I'm sure some will find flaws with what I've written. Fine, don't care. This is from my own perspective,
my point is different people have different ideas, often opposite of each other. The writer needs to take responsibility for his/her own work and make his/her own decisions whether a critique is helpful.