I think that most people don't fully appreciate the complex relationship between the government and the governed. What we've had in the past and are missing now is oversight. This comes in multiple forms: congressional oversight, regulatory oversight, judicial oversight, etc. Ultimately, if those methods failed, we always had the media to provide oversight - which is why until the last 20 years or so, freedom of the press was considered sacred.
Real harm is happening today; it's just hard to prove. The entire reason there is no evidence that the NSA machine has been used for nefarious purposes is because they've designed the programs to avoid congressional oversight. The NSA/CIA deliberately and consistently lied to congress and the judiciary about the scope and legality of its programs. When the media stepped in because other oversight had failed, they were treated as traitors and threatened with jail.
The reason for all this oversight is because totalitarianism is what creeps in when you have no oversight: in the absence of a watchdog, government will naturally work to make the task of governance more efficient. The most efficient method of government is totalitarianism, so their efforts will converge to this. This is absolutely the slippery slope argument: once oversight is removed, the government can do whatever it wants and threaten anyone who disagrees with treason.