Crowler and Peri make very good points. If you are accused of terrorism you could well be tried in a secret court where you aren't allowed to know what the evidence against you is. How can you defend yourself?
Add that to surveillance of every email you send, every Facebook post, everything online, and consider the danger that something may be misinterpreted - look at the guy who got sent to prison, FFS, for posting a stupid joke about Robin Hood airport, that anyone but a crusty old judge would have known was a joke...
And McCarthyism is a real danger - for anyone who doesn't know, in the 1950s, thousands of Americans were harassed, threatened, had to flee the country, even imprisoned on trumped up charges of being 'unAmerican'. He wreaked enough havoc - imagine what a McCarthy could do today, with such surveillance. Even a US McCarthy on UK citizens, given this surveillance, and as the UK makes very little attempt to stop the US extraditing our citizens.