Strong arguments against mass surveillance
Incompatibility with democracy
With mass surveillance, the government essentially puts its citizens under general suspicion. This profoundly disturbs the relationship between citizens and their government, and it creates a cascade of distrust.
Mass surveillance is the inversion of the democratic principle which requires that citizens oversee the government. Hence, mass surveillance is not compatible with democracy.
Mass surveillance mainly affects law-abiding citizens because criminals will go to great lengths to avoid surveillance, and they can resort to means of communication that can't be (or aren't yet) monitored, which isn’t practical for common citizens.
Counter-terrorism is supposed to serve as justification for mass surveillance. In the past, however, mass surveillance has not proven to be a viable means to prevent terrorist acts.
Undesirable consequences and unnecessary risks
If someone knows to be subject of surveillance, they will change their behavior and, ultimately, their thinking. Mass surveillance will therefore lead from self-censorship of individuals to a conformist society as a whole.
Mass surveillance renders confidential communication impossible. Lawyers and doctors can't communicate with clients or patients, respectively, without violating their legal duty of confidentiality.
The data collected by means of mass surveillance can be stolen. With each duplication, the risk of unauthorized access to private data increases.
The data collected by means of mass surveillance can be misused. Even if misuse doesn’t seem likely at present, the accumulated data can be misused in the future.
It's still uncertain what kind of conclusions about a person can be drawn from the data – and especially the metadata – that's collected by means of mass surveillance. Thus, even supporters of mass surveillance don't know what they would get themselves into.
Rebuttal of “I have nothing to hide” argument
“If you've got nothing to hide, you've got nothing to fear” is not a valid argument because privacy is a value in and of itself. If privacy were only to be protected if there is something to hide, it would be useless.
Those who advocate mass surveillance on the grounds that they have nothing to hide must, according to this logic, be prepared to let the government install surveillance cameras in their bedrooms (or else they would have to admit that there is something to hide).
Any author of data who hasn't violated a law should, as its owner, be able to freely decide what happens to this data. Unsubstantiated storage of personal data by the government is tantamount to expropriation.
Proponents of mass surveillance argue that those who aren't guilty of anything have nothing to hide (and therefore can't oppose to surveillance). But those who aren't guilty of anything shouldn't be subjected to surveillance in the first place.
Rebuttal of the justification by way of increased security and counter-terrorism
No level of surveillance can ever guarantee complete security. The demand for more surveillance on the grounds of an increase in security has therefore no limit.
Counter-terrorism is supposed to serve as justification for mass surveillance. However, the terrorists’ goal might be the subversion of our democratic society, and in abandoning privacy, we help them reach that goal.
Counter-terrorism is supposed to serve as justification for mass surveillance, but fighting terrorism by trying to prevent all terrorist acts is fighting the symptom, not the cause. To combat terrorism effectively, its cause needs to be addressed.