One of the the most powerful things I've ever seen on Youtube is The Sagan Series by user damewse. Designed as unofficial commercials for NASA, they use sequences of spoken word from Carl Sagan's Cosmos series cut with a specific piece of music and a variety of assorted footage. The first video in the series, “The Frontier is Everywhere” is one of the most impactful of the seven. Because the piece is a visual montage, it relies heavily on artful editing to drive its points home. The editing choices themselves are based in the two elements of audio in the piece: Sagan's voice, and Michael Marantz's “Earth: The Pale Blue Dot”.
The cuts in this sequence are very reliant on the tempo and rhythm of the music. Nearly every cut in the first third of the sequence occurs on the strike of a key on the piano. This has the tendency to draw attention to the cuts, but not in a way that is distracting. The whole point is to draw attention to the visuals, which are often beautiful shots of the landscapes of Earth from the surface and space. As the music becomes more flowing, so do the cuts. By this time, Sagan's monolog is the more important element, so the timing of the cuts better match the subject of his discussion.
As far as the subject of the visuals themselves go, they more or less match something Sagan is talking about at the moment they appear. For example, the image of Earth from orbit on Sagan's words, “terraqueous globe” moves to footage of rioting and burning oil fields when he says, “we, who cannot even put our planetary home in order.” This immediate relation between Sagan's words and the images seen enhances the effect both have on the viewer.
The video only cuts to black twice, both are fade outs, and they occur at roughly the one and two third marks of the sequence. The interesting thing is they both happen after Sagan asks a rhetorical question before continuing in the next section to answer it. The effect these fades create is that of markers in the piece, when a turn is about to happen. They work well as small segues, and makes the viewer even more interested in what will come after the fade in from black. It's no wonder the video not only is popular on Youtube, but one of my favorite videos on the site.