Of all the departments in video making, I consider editing as the most toughest part and the time consuming one. You can completely tell a different story based on the same raw footage. It is the place where you can correct the mistakes or you can make new ones. Essentially, an editor can make or break a story.

Choosing right music is also important. The combined effect/impact of audio and video is much greater then the individual effects combined. Unless you have a complete sound track available or you have access to a musician who can compose the background score, it may be necessary to edit the audio as well. Audio editing, sound engineering can be viewed as digital signal processing. This is what I did in Jana Gana Remix

One can easily learn various editing techniques, but using the right tools at the right time is much of an art than of science.