As short and simple as it is, this is still an interesting historical landmark, as one of the first movies to be surrounded by public controversy. Some of the other early movies are remembered for the initial surprise they caused (for example, the fear that some audiences felt when they first saw footage of a train coming towards the camera), but the reaction to this movie was different.
Given the accounts of the reactions that it caused, the footage itself seems surprisingly innocuous. The participants in "The Kiss" are neither young nor attractive, and their feelings towards each other seem more affectionate than sensual. That it caused such comment in its time no doubt speaks in part to what that generation was concerned with, but even at that, surely most persons had seen this kind of behavior before.
What made this different was that it was projected on a large screen for all to see, and that an intimate moment had been captured in a form that could be preserved forever and replayed over and over. Unlike a stage scene, a movie is never really over and forgotten, since audiences can still see it many decades later. Also unlike a stage scene, a movie camera could capture the scene in a (medium) close-up, bringing the viewer much closer to the kissing couple.
A movie also captures the entire sequence of events, so that the impression of what is happening is fleshed out in its entirety, making it more memorable than even the most well-chosen moment for a still photograph. All of these thoughts may not fully explain it either, but the fact remains that, though there are certainly some different things that other media can do better than cinema, this was an early example of what movies can do to a degree that other art forms cannot quite rival.
