It is a tricky business editing other's work. I have come up with a few all-around rules to follow.
1. First of all say what's good about the work before delving into negatives.
2. Make sure you understand the voice before you pick it apart.
3. Honor style and don't attempt to insert your own into someone's work.
4. Make sure you understand what the writer needs from you before you start. This is of the utmost importance!
There may be other rules to add to this list but I'll start with these. Anyone have any other suggestions?