It was wildly popular back in the day. The kind of popular that gets immediately picked up and imitated and/or used by other commercial enterprises, and is played to death everywhere from dance clubs to Target commercials.
But the song was pretty good originally. While I certainly can't speak for everyone, I liked it because of the melding of Freedom Williams' rhythmic monotone against Martha Wash's powerfully melodic voice. (That ain't Wash in the video, by the way. The producers thought she was "unmarketable" because of her size and got a skinny chick to lip sync for it.) It was the disparate contrast in styles that, when brought together, made for audible interest.
Try to imagine, for example, listening to Williams' lyrics without the backbeat or the music or Wash's vocals. It'd be dreadfully dull precisely because he has very little inflection in his voice and relies primarily on rhythm. In and of himself, there's little contrast. Wash's vocals, on the other, do have some changes in pitch, tone, and volume, making those somewhat more interesting. More contrast, more interest.
One of my favorite bands, Genesis, used to use contrast like that frequently. I heard at one point that keyboardist Tony Banks in particular really tried to put that concept into a lot of songs because he understood that a piece of art is more interesting when it presents itself in contrast to something. And if you can bring that contrast within the piece of art itself, it's then independent of contextual surroundings. Witness the slow-moving and quiet opening of "Fading Lights" against the more caustic feel it brings forth with much heavier drumming, staccato keyboarding and distorting on the guitar about a third of the way in. Which then resorts back to the original ballad for the final third...
And why am I discussing music on a comic book site? Because, of course, contrast is equally important and significant in visual arts like comics.
Here's the opening splash page from Fantastic Four #51, "This Man, This Monster"...
The issue is considered a favorite among many comics fans. One of the reasons, I suspect, is that there are several types of sharp contrasts in the book.
First, of course, is the Thing himself. He was very much designed as a character to look monstrous, but act exceptionally heroically. Even when he reverts to human form, as he does in this issue, he's not exactly the most handsome fellow.
Second, the series by this point was being drawn by Jack Kirby and inked by Joe Sinnott. Jack worked incredibly fast and turned out extremely imaginative ideas. However, the technical merit of his illustration skills was often sacrificed. His pencil work is powerful, certainly, but in a very raw sense. Sinnott comes in with clean and precise brush work over the pencils and makes Kirby look much more slick and polished. I seem to recall reading an interview with Sinnott where he noted that he basically just traced Kirby's work, but straightened up the lines a bit. And understatement, certainly, but Sinnott's solid blacks contrasted nicely against Kirby's excited sketches.
Third, in this story in particular, Lee and Kirby balance the fantastic with the mundane. Reed Richards goes out to explore the Negative Zone for the first time utilizing room-sized machinery that looks like it could've come from a surrealist painting. His exploration is wrought with danger and almost invites death. But the underlying story is all about the Thing's inherent humanity. The climax of the story is quiet, somber and reserved in opposition to the explosive panels showing Reed's brush with death.
Finally, the individual pages of artwork themselves (such as the one shown above) provide contrasts using light and dark elements. Particularly noticeable here are the shadows on the Thing; his left side is largely covered in deep, rich shadows. His right side with more light on it is then offset by the black silhouettes of the buildings behind him.
That's one of the factors that made the Fantastic Four work. Quiet moments of contemplation opposite world-devouring antagonists. The ultimate man of high science working alongside one of the most down-to-earth men on the planet. The main villain vacillating between science and sorcery. Trying to have a simple, family life but also having the most fantastic powers imaginable. The book resonated with readers around these (and other) contrasting elements.
The moments of high energy, excitement and drama in a book are all well and good, but only when put in relief against the slower, quieter moments. Creators shouldn't need to "shake things up" so much as just not keep resorting to the same tropes (both with regards to story and art) all the time with nothing for readers to balance it against.
Now, add some contrast to yourself by getting your ass up out of that chair, flipping on some favorite tunes and boogiein' down!