This is a far too long response to a post by Adeline Koh.
I agree with Ted's point that DH is a social category more than anything else, but, as he acknowledges, such social categories are consequential. I've argued before that the search for "the most digital digital" is basically intellectually doomed. But I don't think that's the question Adeline was asking—I think she was already asking the social question. (Matt Kirschenbaum gave a social answer.)
In most cases in the humanities, there isn't that much disciplinary boundary-policing; people usually care more about whether the work is good (for what it is) than whether it's "modernist" or "eighteenth century" (a century reputed to be quite long!) or, to use Ted's examples, Marxist or New-Critical. Thus "[t]he ideal PMLA essay exemplifies the best of its kind, whatever the kind." To the question of "what kind of scholarship is this?" PMLA literally says "whatever"!*
Weirdly, though, when it comes to digital humanities, the digitalness (how digital?) matters a lot. In some quite consequential institutional settings (hiring, fellowships and grants, tenure), what kind? matters for things marked "digital" where, for other things, the operative question would be how good? (for widely varying definitions of "good," of course).** It's nice to say, "focus on the scholarship, not on whether it's DH" (#4 above). But there's a reason people focus on whether it's DH: largely through the urging of digital humanists themselves, digital work has come to be seen as warranting an entirely different evaluative system from "traditional" scholarship, so that the question of how good? depends on the question of what kind? in the first place. So in practical terms, "how digital?"—philosophically incoherent as the question is—often serves as a proxy for "how good?," and even if we think it shouldn't matter we've set it up so that it does (and not entirely without reason).
Matt's social answer to the "how digital?" question—tautological or recursive, depending on how we prefer to read it—is that "It is DH if it assumes value within a community of practice that 'does' DH."
But Adeline's question was posed in the specific context of putting together an introduction to DH for people who need one—who have heard of this "digital humanities" thing, do not [think they] do it, and would like to. If they're in "a community of practice that 'does' DH," they're not aware of it. Adeline's task is to inform them of how they might create or join such a community of practice. Under what circumstances would creating an online journal constitute such a thing?
So I think Ted's right; it's a social question. But it's a social question that matters for social reasons that can't, I think, be disavowed without abdicating responsibility for the institutionalization that was so ardently fought for (resulting in an "eternal September" or "DH II" that long-time practitioners are now declaring uncomfortable—y'all, what did you think was going to happen?).
Is an online journal "DH"?
I think Matt's social answer to this social question probably comes closest to the mark. But I also think what he's describing is unfortunate. It would be better, I think, to examine why the social question—how digital?—keeps mattering, so we can figure out how to make it matter less.
* But: as far as I know, PMLA is not set up to publish a database.
** I'm glossing over some local circumstances of boundary-policing—like, we all know that C19 has a vision of American lit scholarship that's different from ASA's or ALA's.