Matt Yglesias jokes about how Bill O'Reilly's 17th century analogue would have fought against the War on Christmas by "shouting monarchist slogans." It's just a joke, but there is an interesting element there that I've been thinking about recently, which is that "conservatism" is a pretty old idea that you can observe through history, despite the fact that what conservatives of any given era have taken their conservatism to stand for has varied considerably.
Basically (and I don't think this is a novel observation), there has always been an identifiably conservative faction in society, but it has never been tied to any specific ideology. Conservatism, in any era, has argued that society's hierarchical structures should always resemble those of the current day, or possibly of one generation (but no more) previous. In the 17th century that might have meant supporting the divine right of kings, while by the 19th century it was enough to defend the institution of slavery.
It follows that there is no common principle uniting the conservative mindset of one era to another: one can recognize the constancy of the conservative attitude over time, the goal of preserving contemporary hierarchies; but an apology for landed aristocracy as a matter of principle would be as valid today as it ever was. Which doesn't mean there's no valid argument to be made in support of conservatism at all, just that it will be a practical rather than a principled one.
My best formulation of such an argument in defense of existing hierarchies starts from the counterfactual conditional that, if all the "stuff" in the world (money, food, natural resources, talent...anything and everything that contributes to a better quality of life) were to be divided evenly among the entire world population, then the amount of said stuff accruing to each individual would be insufficient to lift that individual out of abject poverty and its associated misery. That is, a purely democratic allocation of the world's wealth would leave everyone in miserable destitution.
Wouldn't a situation wherein a minority of the world population lives in relative comfort, at the expense of everyone else, be preferable? The majority are miserable, sure, but they would be miserable under the egalitarian alternative anyway: at least this way, somebody gets to be happy. The implication, then, is not that divine monarchy or primogeniture or limited suffrage are, in each of themselves, the linchpin holding this state of inequality together; but that any capitulation to democracy might trigger the collapse into the general democratic allocation of stuff that has already been judged unacceptable.
A leftist response to such reasoning would not so much dispute this argument as dismiss it as unjust: if indeed comfort for any can only be achieved on the backs of some others, then achieving comfort (even broad comfort) cannot be the animating goal of a just society. And I think this leads to a corollary regarding the conservative attitude, which is that justice is not a component of the conservative vision for society. (Or to put it another way, the conservative notion of justice relies on context: that which supports existing social hierarchies is just. So at one time it is just that the monarch own everything, while at another time it is just that each individual's private ownership of property is sacrosanct.)
One final note is that despite the long time spans used to illustrate the historical incoherency of supposed conservative principles, this is not a semantic quibble that can be sidestepped with anything along the lines of "well I wouldn't have been a conservative back then." A large number of today's mainstream conservative leaders, after all, are on the record having supported South Africa's apartheid government in the 80's, though none today endorse apartheid as a reasonable solution to ethnic conflict (e.g., none of the considerably heated opposition to Jimmy Carter's Palestine Peace Not Apartheid proposed anything along the lines of "peace by way of apartheid").
I think this is the cause of much disconnect between conservatives and non-conservatives. It's compounded by conservative rhetoric, which does make nominal appeals to principle; but in practice, conservative principles are fluid, adapting to fit the present circumstances in service of the real conservative project, the resistance of democratization.