When you critique other people’s work, it’s hard not to compare what you’re reading to what you would have done. “He should have started with a quote,” you think, shaking your head. Or “Oh God, not the passive voice!”
This attitude will not make you popular with your colleagues. More to the point, it doesn’t serve the project.
If the writer is guilty of poor grammar or word choice, awkward phrasing or other laziness, you need to call him on it. But before you do, think about what’s really bothering you. Is it the way he writes? Or the fact that he doesn’t write like you?
When the writer does make a mistake, your criticism should be constructive. Scrawling “AWK” all over the piece won’t help. Neither will rewriting it–he’ll be insulted and you’ll be angry that you had to spend so much time on someone else’s work. Try to offer concrete suggestions (“rephrase as an active statement,” “eliminate repetition”) and vent your frustration somewhere other than on the page.