They have to do with the premises from which he is examining libertarianism.
For example, when he says that libertarians wish to force their ideology upon non-libertarians, that may make you stand up and cheer, until you realize that what he means is simply that libertarians are not willing to be forced at gunpoint to pay for the leftist social experiments his "non-libertarians" are intent upon.
You see? Taking his argument out of its socialistic context and putting it in one that is more familiar to you makes it evaporate and drift away on the wind. Because he believes that it is socially unjust when people attempt to maintain control over the fruits of their labor, he sees it as coercion against the people who would otherwise benefit from the redistribution of those fruits. But that view is only possible in that context. If you are a supporter of property rights, then you cannot argue that libertarians wish to coerce non-libertarians into anything.
Another example is his oft-repeated "bad truck" analogy. He's talking about a situation where market forces led to people not buying a poorly-designed and -built product, meaning that its manufacturer went out of business. He sees this as a bad thing, because of his socialist context. What that must unavoidably mean is that he believes that somehow people should have been coerced to buy this inferior product in order to keep its incompetent manufacturer in business. Is that what you believe? If not, then all the attacks he mounts with this "bad-truck" analogy similarly evaporate and float away.
That's what people on this thread mean when they say that this writer's prejudices invalidate his points. They mean that all his attacks on libertarianism are founded upon the premise that his socialism leads him to hold different objective standards of good and bad than most Americans would. That means that any defense of libertarianism aimed at this article would perforce turn into an attack on socialism: and probably most of the people on this thread already agree that socialism is bad. Therefore, such an argument is a pointless waste of time.
If you disagree with me, please pick out what you think is his absolute best criticism of libertarians, and I'll have it out with you, and you'll see what I mean.