I wrote about giving feedback on other people's work in an earlier post and I stressed the need to emphasise the positive.
Over at Critters there is an article on how to deliver a critique diplomatically which puts the stress on how you give the feedback. The most important point is that unless you phrase the critique in such a way that the other person can receive it you are wasting your time.
The main thing you have to do to make a critique easy to receive? It seems that it's easier to take notice of a critique when the critter makes it clear that everything they are saying is their own opinion. So plenty of "I think" and "I feel" and "it didn't work for me" and none of "you must" and "the rules". And definitely no attacking the author!
It's a good article and I recommend it to you.