In general, I think it's right to understand major historical developments as the products of economic and social forces rather than the personal traits of particular individuals. Of course, those forces work through individuals. But the idea is that if, say, Markos Moulitsas hadn't been born, there would've still been a big lefty website a lot like Daily Kos except it'd have a different name and a different person (perhaps with a similarly acerbic style) would be running it.
Insofar as individuals matter, it's starting to seem to me that a lot rides on their below-the-neck physiological capabilities. You pick a Supreme Court justice in 1975 based on the prevailing selection criteria at the time, and you're going to end up with John Paul Stevens or someone a lot like him. Whether that guy is still on the Supreme Court through all eight years of the Bush Administration, keeping Roe from being overturned and otherwise affecting national policy in various substantial ways depends on whether he's able to stay healthy long enough.
Supreme Court appointments are probably the most egregious case in our political system, but there's plenty of other places where things look similar. The way that Senate Democrats have been picking their committee chairs, based on seniority, has a similar effect. Whether your committee chairman is from Montana or Massachusetts depends on which person elected thirty years ago could live a long, healthy life and generate enough seniority.
It would be good to reduce the impact of these kinds of factors on our political system, and put things into the hands of human beings rather than the health lottery.