RE: LIBERAL COLUMNISTS. Even if it's true, as Kate says, that the median liberal is "more inclined to examine the many facets of a topic, to wade into the subtleties of an argument, explore the finer points, and concede to the possibility of conflicting evidence or opinion," it's certainly not the case that all liberals are trapped in a labyrinth of subtlety and searing self-criticism. So there should be more than enough to field a few columns!
Indeed, the blogs are full of writers perfectly capable of torching a town to roast its single conservative inhabitant, and any one of them could be conscripted to write a column. Molly Ivins, Michael Moore, Al Franken, Jim Hightower, and many other liberal writers manage to forcefully advocate progressivism without getting trapped in a handwringing-feeback-loop, and they do it -- or in the case of Ivins, did it -- with style, wit, and humor. Hell, David Sirota, whose skin would sizzle if it ever came into contact with nuance, is now writing for Creators Syndicate. So these writers exist -- I think the question remains why they aren't more broadly distributed throughout the nation's newspapers.