We were taught at school that pointing out the good is just as important as pointing out what could be improved with a script. (We never say "bad". School = political correctness. Ha! Right!)
If you tell a writer everything that's crappy about his/her script without pointing out the good bits:
a)they'll resent you and not listen to another word you say, even if your suggestions are good ones, and/or
b)they'll give up. Which would suck, because great scripts are only a result of a lot of work.
If you show them that you can appreciate everything they did right, they'll be more willing to appreciate that you know what you're talking about, and take your suggestions for improvement.
If you want to insult someone effectively, compliment them first
so they know you understand them. They'll take the insult soo seriously.
What I meant to say:
Encouragement is underrated.
When I hand in my story-editing notes, even the positive feedback on my positive feedback is remarkably uplifting =)