In my experience, there are two common dismissals of the old school movement. The first is semantic, claiming that the term "old school" is so devoid of a commonly accepted understanding that it's meaningless. The intention behind this dismissal is to prevent discussion by undermining the common meanings of terminology. Needless to say, I don't think much of this point of view, because, while I'll concede that lots of people do in fact use "old school" in a wide variety of ways, that doesn't in itself say anything about the meaningfulness of the term as it's generally used among players of these games. It's a rhetorical trick, not an actual argument.
The second dismissal claims that old school is "a feeling" and can be divorced both from particular games and particular mechanical designs. The intention behind this dismissal is to claim that one can play any game in an old school fashion, regardless of its vintage or rules. It's an attempt to divorce the animating principles of the early hobby from its mechanical foundations. This is a somewhat more sophisticated dismissal, but, ultimately, it's still a rhetorical trick rather an argument. It's an appeal to an ill-defined "spirit" of the old school as a means of undermining attachment to any particular old school game.
That's precisely why I've never bought into the notion that the old school is just a feeling: it makes rational discussion impossible. If the old school is just a feeling, then it's purely subjective and beyond our capacity to argue for. It's a mere fancy rather than the product of serious thought. Now, I don't' want to argue that the old school isn't a feeling, because, on some level, there definitely is an "emotional" component to it -- but that's not all that it is and I don't think it does the old school movement any good to tacitly accept the idea that "old school-ness" is primarily felt rather than apprehended.
There is in theology a term that seems apropos: indifferentism. Indifferentism is the notion that all religions are equally good and valid provided that one practices them with proper intentions. To my mind, the idea that the old school is primarily a feeling is a kind of ludic indifferentism. No doubt many proponents of old school-as-feeling do so out of a genuine desire to avoid One True Wayism, which is certainly laudable. The problem is that, by arguing for a primarily emotional understanding of the old school, one quickly reduces all arguments to arbitrary preferences. That I consider, say, Swords & Wizardry a game truer to the old school than Exalted or 4e is nothing more than my personal feeling on the matter, a feeling that's impossible to articulate rationally and that others can feel free to dismiss without having to understand just what I mean when I say this. Likewise, when a player of such games claims he's doing so "in an old school style," I have no recourse but to accept him at his word and move on, because no argument could possibly be offered to disprove his feeling that he's playing an old school game.
Let me stress again that I am most emphatically not arguing against the notion that feelings and intuitions have a role in coming to an understanding about what the old school is and is not. However, I feel "I'll know it when I see it" is inadequate and contributes to the absolute subjectification of the term "old school." Consequently, I think it's vital, particularly now that more and more people are looking at old school games with new eyes, that this community shy away from speaking primarily in terms of "feelings," since that path leads to the chaos of indifferentism.
If one actually believes, as I do, that games like OD&D, Tunnels & Trolls, Empire of the Petal Throne, and so forth offer something unique that no game published in the last 20 years can match, then we ought not to rest our case too heavily on nebulous quasi-emotional impressions. I think there are enough clear, rational, and unambiguous arguments in favor of the old school that there's very little need to invoke feelings at all. More to the point, to resort to feelings is basically to concede the argument before one has even begun, which only contributes further to the mistaken notion that one's liking for an old game system is nothing but nostalgia for one's lost youth. In some cases, that may be true, but it needn't be the case and the continued success of the old school renaissance depends greatly on promoting the unique qualities of older games in a clear and rational fashion.
Otherwise, we really are just a bunch of middle-aged guys clinging to the past.