I would make a distinction between anti-abortion terrorists and right-wing terror groups. The anti-abortion movement is motivated by a religious/moral objection to abortion, not necessarily any devotion to other elements of right-wing politics such as hatred for the federal government. In fact, there are Catholic anti-abortion activists who can be quite progressive on other social justice issues. The same could also be said of homophobic hate groups.
To me the key elements that motivate right-wing terror groups in the US include a paranoid hatred of the Federal government, a virulent fear of international organizations, a belief that the federal government is engaged in a massive conspiracy to eliminate the political rights of anyone who is not a leftist one-worlder, an extreme emphasis on the supreme importance of private property rights, a belief that unfettered individual access to firearms is essential to protect citizens from the federal government, a strong survialist ethic, and a belief that "the system" favors women, immigrants and racial minorities over "regular white male americans".
One other thing. As others have noted, the elections of President Obama has galvanized the fringe right wing. Not only does he personally hit all their red buttons (a smart liberal black person with a suspiciously foreign-sounding name who Europeans like who they they think is moslem and not even a legal american citizen and hence an illegitimate president), the fact that he has been elected twice fairly comfortably reinforces the extremists fear that America as a nation is no longer the America they worship. This is frightening because fringe ideological groups often only become fringe terrorist groups when they have reached the point when they believe they have basically lost the fight in the political arena.