It isn't necessarily the attacks themselves that make terror universal, it's the uncertainly and the effect on daily life. Countermeasures, responses, and panic can be as restrictive and disruptive as the actual terrorist campaign. As the IRA said, "Remember we have only to be lucky once; you will have to be lucky always."
In the United States, we can see the effects of terrorism not through attacks but through our responses. We see increasingly elaborate, lengthy, or invasive security practices at airports and government spending overseas. Some of us are deployed fighting the War on Terrorism, and almost everybody knows someone who is. Elsewhere, the costs are even more palpable, with checkpoints and drastic civil rights infringements.
Governments deem such massive reactions necessary because, as the IRA said, they only have to succeed in one attack to cause widespread fear while the authorities have to block every single one. This is further complicated by the problem of false alarms and hoaxes.
The more thorough procedures are for detecting terrorism, the more likely they are to cause false alarms by erring on the safe side. The same goes for investigating possible calls and threats. Many terrorist groups who wish to target public areas but may not be interested in inflicting civilian casualties, such as many ethnic and nationalist groups, may call ahead with warnings or leave notes. There is no code word to identify legitimate terrorists, however, and actual calls are often low quality, hard to understand, and amateuristic.
During the Troubles, despite the high, steady rate of attacks, there were still many more hoaxes than actual bombings, making the real warnings ineffective. On the day of the King's Cross bombing, for which no warning was given, there were over 100 hoax telephone calls forcing the evacuation of three other stations. This adds to the uncertainty mentioned in the last post. The state doesn't know if there will be warnings before an attack and when, nor does it know if the warnings it receives are legitimate. As a result, real warnings, no warnings, and hoaxes all add to the terror and the effect of the campaign.