Everyone is subjective, as is "improving" or "good"...a helpful editor is important to give a different and critical view of things, to see a work from a new perspective and give feedback the author wouldn't have thought of herself. This is how we balance our own subjectivity. Beyond this, having done both, I can truly say that editing is a very different skill from writing.

And of course, there's always grammar, which isn't subjective at all.