Despite my occasional jokes about being a Wielder of the Red Pen of Doom, I'm an easygoing person. I prefer to take a minimally invasive approach to editing: I will not allow my pet thesaurus to sneeze all over your manuscript. I will not attempt to force my own voice into your writing. If sentence fragments are part of your narrative style, I'll help you make them work for you rather than tell you to get rid of 'em. It's your story; I'm just here to help you tell it.
In addition to a thorough understanding of grammar, punctuation, and other mechanics of writing, I have an eclectic knowledge base that comes in especially handy when editing science fiction or fantasy. I read a lot of science fiction and fantasy, so I'm familiar with normal tropes of those genres, which means I can give good advice on how to follow those tropes or subvert them, depending on the author's intentions.