One thing I often see is people not understanding the difference between secrecy and privacy. They ask why it matters if you're not doing anything wrong. A UK government minister actually said "if you have nothing to hide, you have nothing to fear", and then backpedaled when someone pointed out they were quoting Joseph Goebbels. The analogy I've seen is simple: I'm sure you don't do anything illegal in the shower, but I'm also pretty sure most people would be uncomfortable with a law that required you to have a police officer standing in you bathroom with a video camera to record you showering, just in case.

The other thing is the assumption that any information about you that the government actually has about you will only be used against you if you commit a crime, in which case you'll deserve it - if you're not a bad person then it's fine. This is a double fallacy.

First, we've seen that information can be used to do all sorts of things regardless of wrongdoing - if someone knows enough about you, they can use it to manipulate you. I don't mean blackmail or whatever, although that's an option. I mean that with a clear enough picture of your preferences and biases and habits, someone can tailor their actions and information to your psychology and make you think whatever they want you to agree with.

Second, it assumes that you won't ever commit a crime because crimes are bad things and you're not a bad person. This overlooks the possibility of you being mistakenly accused while innocent, but more importantly it overlooks the possibility that the government will change into something that holds different moral values to yours. Even in the modern world we've seen places outlaw abortions, or criminalise homosexuality, or pass laws on what religions you're allowed to follow. If that happens in your country and you find yourself on the wrong side of whatever arbitrary line they've now drawn, you may regret giving them so much information about you - information that lets them identify you, prove that you broke their new rules, and ruin your life in so many ways.

The default principal of any exchange with governments, businesses, or any entity taking your information should be to give as much information as is required for them to perform the operation you're requesting of them, and no more - and wherever possible to only engage with those entities that you trust to have that information; a trust that they earn by a verified and unbroken track record of ethical and trustworthy behaviour.