Machines categorising you based on your behaviour, without your knowledge nor your consent. It's not so bad when it serves you ads (unless it sells alcohol to alcoholics), but there's no telling what similar algorithms would say about you in the hands of a rogue government. They can find vulnerable people, people who hate certain people [0], people who talk to certain people or hold certain ideas.
What makes it even more terrifying is that machines can categorise people much faster, based on a much broader set of information. It's not just informants and paper reports, but computers processing and connection millions of data points.
I'm bringing all of my data together[1], and the result is a graph of every place I've visited, every conversation I've had, everything I looked up, every book I've read, every transaction I've made, every video I've watched and everyone I've talked to. There's even more data about me in the wild, and if you combined it with other people's data, you could figure out even more about my every move.
It's a good thing that the Stasi was a few decades early.
[0] https://www.propublica.org/article/facebook-enabled-advertis...