Alright, the long and short of receiving critique was addressed a few chapters/sections back. I think, to balance everything out, the matter of giving critique should be addressed.
Sometimes, I think we're afraid to critique someone's work because there are so many negative connotations associated with criticism and critics and so forth. We're afraid of offending someone with our "judgement." One of the definitions of the word "criticism" is "an analysis of a work of art or literature."
So, if we want to get picky, every analytical comment is, indeed, criticism.
When I try to address something that's kind of iffy, I usually try to also address something I liked about a piece. Then I give a usually-gentle nudge.
For example: let's say I read over a piece, and it's very exciting and engaging, but the spelling's absolutely horrid, I might say something like this: "I'm picking up on a great deal of action here, but you might want to double-check some spellings."
Or, if there's something you could suggest to make the piece better, say so. Maybe there was a particular word you think would fit a bit better in such-and-such a phrase, or perhaps this other bit should be extended a bit, or that other part could be curtailed.
One should be careful with tone. It's not the greatest if someone comes off as accusatory, or if it sounds like (s)he's looking down on the other person. We're all in the same boat, and should treat each other as such.
Don't think of it like you're criticising someone; think of it as you're suggesting things that will bring their art to a new level. Because that's what you're doing.