Maybe the real issue here is that, as was observed, she foot faults 10+ times a match, and they're not called. If this is going to be something they're going to emphasize, 2 points from the end of a match this deep into a Grand Slam is not the time to do it.
To continue the analogy that has been made, it would be like if in the NFL, the DB's had been faceguarding the whole game, and then on a pass to the endzone with time expiring, all of a sudden they call pass interference on him. I think most athletes recognize that different officials call game/matches/whatever a little differently, but the one thing they ask is that the officials be consistent. I think calling it for the first time this deep into the match was a mistake by the official.
That being said, Serena was way out of line. It's one thing to yell at an official, it's another to tell her you're going to shove the ball down her f****** throat, or kill her, or whatever.
Sure, people have done this before, as has been mentioned, without this kind of backlash. But in a more commercial, televised era, people don't want to see that kind of junk any more. Passion and frustration is fine, the occasional argued call with the chair is fine (although there shouldn't be much with the replay camera), and sometimes slight shows of frustration by hitting a ball against a fence, but this was way beyond that. Not only did she yell at the official, but then she didn't quit while she was behind, and went back for seconds.