Editing is the art of tying clips together cohesivley in order to tell amazing stories. What makes good Editing? Cohesiveness, continuity, aesthetics, are all visually present in good editing.
Obviously there are many forms of editing, those for storytelling where story is king and those for a much more visual experience such as music videos.
When your objective is to tell a story then paying close attention to pacing and build up are of much importance. You must recognize what you should rush and what should be dragged. The build up is very important to genres such as horror, those moments of suspense in which the editor is using all his tactics to slide you all the way to edge of your seat are all by design. Learning to generate this tension will separate your work from the rest. It's all about timing and reveal, let the juices drip until the precise climatic beat. This is not only the case for horror holds true for all storytelling, you must structure yourself much like a screenwriter to maximize the effect of your story.
On visual projects such as music video you should look to make a montage that's thrilling quickly paced with constant changes, obviously it should transition properly or it simply won't look right. Use all the angles you have while if possible making color decision that bring variances to the images. Hopefully the footage was shot on log or something such as a flat prores as this will give you a lot of color options. Music video can get boring quickly, therefore it's your job to spice it to the limits, obviously you have to work with the footage you have but get creative go the extra mile and try to incorporate some motion graphics as they are a real modern touch.
Think outside the box..