Bruce Schneier is an expert in security, both online and in real life, and one of the most prominent public voices for measured responses to public threats. Schneier offers three reasons why the Boston marathon bombing does not necessitate tighter security measures.
1 Nothing requires fixing. The fact that the occasional terrorist attack succeeds does not necessarily imply any systemic security failures. That's doubly true when the terrorists act alone and on impulse. There are 700,000 people on the FBI's "watch list." It is simply impossible to closely monitor them all.
2 No new measures are needed. No suite of new police powers or new police technologies will magically keep us safe. Investigation, intelligence, and emergency response are what work best; overly specific security measures that need to guess the target and tactic in order to be effective are generally not worth it.
3 Terrorism is such a rare risk that it's not worth worrying about very much. The most dangerous part of any airplane flight, movie-theater outing, or marathon run is still the drive to and from the venue. By far. Terrorism almost never happens, and we shouldn't let the media's endless replaying of the details fool our brains into over-magnifying the threat.