Terrorists that are political or religious in nature have quite a bit in common in terms of their motives. They have obviously come to the conclusion that the systems in place are either dysfunctional, or will never work to their advantage in terms of their goals, so they have resorted to violence. This is an important (and difficult to reverse) frontier to cross for any group. They also share the characteristic of having dehumanized their enemies, having made the judgement that they are undeserving of life. Very few terrorist groups hit only military or government targets, and seem to believe civilian deaths are at least inevitable and at most, necessary.
In Western society, we make a distinction between religious and political motives. We can thus say that, for example, IRA bombing attacks in the seventies were motivated by politics, and that religion was only a pretext. Yet for many radical Islamists, no such distinction exists, and this, I think, is one of the problems Westerners (including many Muslims) face when trying to wrap their brains around what might motivate one to commit an act of terror. We simply cannot understand their motives in the context of our culture.
According to federal authorities, Ahmed told agents that he would be ready to fight after completing a pilgrimage to Mecca next month.
"On September 28, 2010, AHMED told both [operatives] that he was attending the Hajj this year and that they should all go in order to complete the five pillars of Islam before making the 'top mark' - by which I believe AHMED meant 'becoming a martyr,' " Dayoub said. (Spencer S. Hsu, Washington Post, p. 3)
I agree that many terrorist groups actually aren't so much about religion as the political context that has helped them to flourish and grow. This is as much true for groups like Al Quaida, who claim that theirs is a religious war, as it is for other groups who do claim religion as one of their raison d'etre's but actually seem to be more about politics, such as Sinn Fein in Northern Ireland.
The irony is that many of these groups of terrorists take their own lives along with as many "infidels" as possible, usually contending that they are a part of the Islamic faith, but do not follow the teachings and are not supported by Muslims. Islam is based very much on respect, honor, peace, "mercy and forgiveness." The jihad is not supported by the majority of the Muslims of the Islamic faith. It's kind of like taking the responsibility as US citizens for any of our Presidents who have "gone off the deep end." Most of us are just trying to get by. There are always those extremists in every country to garner a great deal of attention, but do not represent the masses.
One difference may be the willingness to sacrifice oneself in suicide bombings. Islamic terrorists seem to be much more willing to do this than, say, the terrorists of the Irish Republican Army or Hindu terrorists. Members of the IRA may have considered such suicide bombings sinful, whereas many Islamic terrorists seem to believe that such suicides are actually forms of martyrdom.
This is a hard question to answer. It is most fashionable and politically correct to ignore religious ideology, but I do believe that religion plays a role. The nuances of this argument will be difficult to construct, because general principles are difficult to establish, but I believe that religious commitments have a big role. In next wars may have more to do with religion than politics.
What terrorist groups have in common is that they all believe they are right. Usually they are trying to prove something or convince people of something. Individual terrorists sometimes just want to cause terror. They want to scare people, and don't necessarily have a vision.
I think that the main difference has to do with the extent to which religion is seen as a major motivating factor in the terrorism. But I don't think this is due to the religions involved. Instead, I think it's due to the broader circumstances.
With Islamist terrorists like Al Qaeda, there is a strong appeal to religion. By contrast, terrorists in Northern Ireland identified themselves as Catholics and hated Protestants, but the hatred was as much about politics as about religion.
I think, though, that that was because of the circumstances, not anything inherent to the religions. The Irish republicans had a clear political grievance. The Islamists have a political side to their anger, but it's harder to express because it's much less clear-cut than having Ulster break away from the UK.