One point that is often made by those in favour of more surveillance is: "If you're innocent, what have you got to hide?"
I'm struggling to think of examples of things people would want to hide from the state qua state, but the state consists of people, and plenty of these are things that you'd want to hide from other people.
Businesses want to hide their marketing strategies and their business plans from competitors. In any negotiation, both sides - whether that's management and unions or one company and another, or a supplier and a purchaser, or whatever - are going to want to conceal their bottom line from the other side.
At the individal level, no-one wants their current employer to know when they are applying for a job elsewhere until they have an offer in hand.
There are also plenty of things that are morally dubious - not exactly "innocent" - but not criminal and not really of interest to the police, but which people are unlikely to want to be made public.
For instance, if you're having an affair, you probably don't want your wife to find out.
For instance, most customers of pornography don't want that to be made public, and even more so for customers of prostitutes.
For instance, lots of people and businesses make financial mistakes, and manage to pay them off eventually - generally they don't want to create a loss of confidence with their creditors until they are in a position to cover their losses.
You'll notice that the major secrets are financial and sexual - that's because money and sex are two of the most important things to most people - along with their own health and their opinions (political or religious, for example). Generally, a person's opinions are something they express publicly, so there's little to keep secret - though there are closet racists and the like - and your health is not in general something you can conceal, though there certainly are cases - for instance, would you tell your employer if you had been depressed in the past? Would you reveal it on a job application?
I'm trying to collect other examples of legitimate secrets; I think it's key to countering the authoritarian surveillance state that New Labour - especially John Reid - is trying to establish.