I believe it's just the difference between film and video. Film has that softer, more subtle look, while video is very sharp and edgy. Since soaps are shown daily, it would be unreasonable time-wise to shoot them on film.
Lighting also has a lot to do with it. Soap operas don't have time to set up the perfect cinematic lighting for every single shot. They light up the whole room, point multiple cameras at the actors, and shoot whole scenes in one take.
I'm guessing a lot of it has to do with frame-rate and interlacing. The soap at video rates (60 fields/sec interlaced) versus film (24 frames/sec) give totally different looks. I've played around with this with a friend's MiniDV camera (Sony TRV-900). You can record at 30 frames, progressive, and the output is strikingly film-like, particularly in low light.
I've yet to see any real 720p/60, though, to back up my theory. My HDTV won't accept 720p.
Great answers...that is it exactly. They have almost entirely phased out shooting sitcoms on videotape, but in the 70's, almost all of them were done that way. What I think is interesting are the friends that i poll who can't tell the difference. I always say, "the evening news looks different than a movie, because one is shot on video, the other on film'...and they still don't get it.