How can you say it changes nothing, when Snowden's actions clearly are changing things?

You may argue that we already knew what the NSA was doing, but specifics matter. If I have a key to your house (for emergencies), you know that in theory, I could be there every minute of the day when you aren't. But you assume that I'm not doing this. The specific knowledge that I was there, and ate the last piece of chocolate cake, changes everything.

While these agencies are nominally accountable to Congress, and thus their mission is "approved", in practice, it seems they are not. Snowden's whistleblowing is a step in the direction of accountability.

Also, don't think like a hacker, think like a politician. Having the information isn't a simple boolean state. There's a difference between "the information is available" and "everybody knows, and it's top of mind". Or "people have vague worries that X and Y might happen" and "X and Y is happening and it's on a grander scale than you could have conceived".

A policy decision most Americans normally have no say in. It would never have come to public debate before the revelations.

What are you talking about? The fact of American agencies spying on the world has been well-known since at least World War II, and is the stuff of major motion pictures, etc.

There's spying in the abstract, which is known just from the existence of intelligence organizations, and there's spying on specific targets (or everybody), which hasn't been subject to public scrutiny or debate until now.

It's one thing to "know" from movies that someone is being spied on. It's another thing entirely to know from NSA documents that you and people you know are being spied on.

Unfortunately that's a neccessity of the nature of these policies. Are you saying the US should have nothing that isn't publically known?

I'd like to suggest that! We wouldn't have done 90% of the evil, pointless, counterproductive bullshit we've done since WWII, if all aspects of that bullshit had been public. And, not a single genuinely good thing we've done would have been prevented.

Details of military battles could be kept secret during the battles and for a couple of years afterward, and sure we should keep demographic data about private people restricted to only those agencies that legitimately need that data, but those are the only exceptions that come to mind.