The difference is that Dropbox was a product everyone already understood the concept behind--nobody needed five paragraphs of text to understand "it syncs files"--but was sold entirely on the execution, and how much better that execution was than all the challengers who had failed to penetrate the space before it. And you really can't get execution from a speel.
To make a gaming analogy: you can effectively market a JRPG through commercials, because, in essence, what people are buying is the narrative. But you can't market Tetris in a commercial, because what's being bought there is the experience of gameplay. Some things can simply be described; other things can only be shown; and still others need to be played with.
I joined dropbox when I heard from a friend how well-designed it was. I then proceeded to sign up for it _despite_ my frustration at the video-only front page. I never did watch the video.
I agree that using subtitles everywhere is a good thing.