These things are designed to be forgotten, but with the Facebook Timeline, much of your life is all but indelible, published front and center until you go through each item individually and hide or delete it.
That, right there, is why this is so terrifying. Regardless of stalkers and people who browse your past for kicks, this is an insane utility for profiling individuals down to the minutiae of every day life. What's more is that it's a catalog, itemized down to the hour of activity on your Facebook. Yes, this has been available in theory for awhile, but now it's highly accessible and easy to find and scrutinize any moment in anyone's life (that is, if they've been posting to their Facebook). The scary part: a lot of people do.
The geek in me wants to applaud Facebook for the technical achievement of the whole thing, but deep down, this just feels a bit too much like opt-in surveillance (whether that be by the government or individuals).