Much has been written about the event. Folks are either in a dither about it, or folks think it's silly that people are in a dither about it. I'm adding my opinion and analysis to the commentary mill just because I haven't seen anyone else express anything comparable to my perspective.
From what I've observed, the outrage, though often poorly articulated, stems from viewers' expectations. It's a simple matter of the ethics of informed consent when we examine the incident in the context of certain aspects of US culture.
In America, we have ratings systems for what we view. The Motion Picture Association of America provides guidelines for movies; television has adopted a similar system, and even the video game industry self-regulates with the ESRB. These agencies provide a structure by which consumers can make a general determination as to whether a given work is going to contain material they consider appropriate for themselves or those in their care.
People can talk about Puritan sensibilities, and it is said that Europeans chuckle in amusement over our citizens getting so worked up about a bit of bare flesh, but in my assessment, they are missing the point. The issue is not what was exposed, but the environment in which the exposure occurred, and the audience present for it. The Superbowl has been televised for many years, and statistics indicate that a third of the American population tunes in.
Despite the fact that I personally remain baffled by football's entertainment value, I respect the right of sentient individuals to choose their own entertainment, as long as it does not tread on the rights of others and as long as all parties involved have consented with adequate data to make an informed decision. Although detractors legitimately decry the sport as violent, that violence was nevertheless expected and anticipated by the viewing public. The nudity, however, was not a customary part of the annual broadcast, which a large percentage of the US population defines/perceives as "family" entertainment. Even if I find it a bit weird, that's their autonomous evaluation. However, they did not consent to view nudity, and they do not include such in their definition of what is appropriate for their Superbowl viewing. For whatever reason, nudity on the main, non-cable channels is a rare thing, and it is prefaced by a disclosure notice. No such notice existed for this telecast.
The members of the audience have the right to choose (even if some of us think it's a bad/silly/unenlightened choice) what and what not to view, and it is that right that was violated by the "costume reveal," as Jackson has called it. Parents were forced into the uncomfortable position of explaining the incident to curious children viewing alongside them. If a football player had experienced an injury during the course of the Superbowl, that would have been something that a viewer could have reasonably expected to have to explain, and formulated a response in advance.
Many have speculated on Jackson's motivations: attention for a new album, an attempt to move the media spotlight off her brother, Michael, in some sort of attempt to aid him, and so on. Some say it's an accident, some say it was intentional. However the situation occurred, the end result was that the audience was exposed to something that, in their view, was offensive material, and they were exposed without their consent. They didn't like the content of the surprise, and they have expressed that view. If I'm about to view something shocking, I like a little notice. That's not unreasonable, and I don't consider it unreasonable for others to want the same.
Those who castigate football for violence and cheer the visibility of the human body are often the same people who support the right to violence in a consensual sexual context. This is logically inconsistent: both are voluntary. The players are informed of the risk of injury when engaging in this combative activity: the term "consenting adults" applies to both bedroom and commercial sports, as it were. One cannot have it both ways. Like it or not, since the viewership did not have the opportunity to accept the change to the standard content of the broadcast, their disgruntlement is justified over the supposed "wardrobe malfunction."