It was a great post until the thinly-veiled Arendt-esque statements about "morally evil 20th century regimes that threaten our precious liberalism" came around, then in became questionably myopic, which is ironic because it's an attack on someone else's supposed myopia.
The comment wasn't about threatening liberalism, but about threatening free speech and diversity of opinion.
To the extent that liberalism was married with democracy (i.e. partially at best), I do believe that its 20th-century manifestations offered a degree of protection for the above values. But the picture was far from entirely positive, with its gross inequalities of wealth and power representing very serious limitations. In conditions where some opinions have almost infinitely greater reach and influence than others, and where liberal democracy in one place can offer support for absolutist authoritarianism elsewhere, we're clearly talking about a system that's a very long way from the ideal.
The regimes I was referring to, however, are those in which the establishment of ideological purity was a core aim of state policy, and the full machinery of state was brought to bear on dissenting voices. The ruling parties in China and the Soviet Union were certainly guilty of this in their worst moments, although, setting aside the sheer scale of their respective empires, the depth of repression over time was somewhat less severe than elsewhere. If we're looking for the worst examples, the Nazification of Germany and the Khmer Rouge period in Cambodia are the ones which jump most readily to mind. With reference to those latter cases, would you disagree with my contention that 'the narrowest viewpoint spawns the most monstrous policy'?
Edit: I was going to say something about theocracy here as well, but I think I've blathered on enough for this evening.