likesdeladies really hit the nail on the head.
The biggest reason, I think, it's such a big deal is because it was Janet's actual nipple that was shown (I don't remember if it was mentioned here, but there is a digitally enhanced photo that shows it was a nipple ring, not a covering). That technically classifies the Superbowl half-time show as softcore porn. On public access TV. If it wasn't for that fact I doubt the FCC would be involved at all because junk that this has been done before only the actual nipple was covered.