First of all, I'm not going to repeat the well-known idea that stricter anti-terror laws act as a "clarion call" to other fanatics. This has been said before. What interests me is this from a policing point of view.
In Al Gore's excellent book "The Assualt On Reason", he mentions that the 9/11 terrorists were already known the the FBI and other policing agencies, and were bring tracked using already perfectly well-functioning methods. The reason they were able to cary out the atrocity was because simply through police incompetence, not because there were lack of laws existing to help catch or prevent them. And from a policing point, this is the crucial issue: effective anti-terror policing comes through sheer hard work at infiltration and detection of threats. Creating more laws makes policing more complacent, not less, because it changes the focus of police's attention to merely minimising the risk, rather than seeking out the root cause. Police say they have prevented dozens of potential attacks through traditional infiltration and tracking; they almost never prevented an attack by relying on the new powers they have through more anti-terror laws.
Another point about crime prevention; creating all these new anti-terror laws may seem to work in the short term as a way of making it more difficult for terrorists, but in the medium and longer term, it just forces terrorists to up their game and get smarter. In other words, anti-terror laws make terrorists think smarter. After 9/11, restrictions were placed at airports: the result was the shoebomber. Then there was the "liquids" plot, that resulted in the liquids restrictions at airports. Then there was the more improvised attempted attack over Christmas. So, well done, people in charge; every law passed aides the evolution of terrorism.
I just wish these people had thought about that, before they thought about all the new powers it would give them.