It's certainly true, and it's easy to see why when you look closely at the editorial:
The biography that voters care about is their own, and they want to know how a candidate is going to improve their future. That means offering a larger economic narrative and vision than Mr. Romney has so far provided. It means pointing out the differences with specificity on higher taxes, government-run health care, punitive regulation, and the waste of politically-driven government spending.How exactly can Romney give more specificity on "government-run health care"? On "punitive regulation"? They might as well have asked for the Mittster to be more specific on Obama's apology tour and, I don't know, Kenyan birth.
Andrew Sullivan personalizes it and argues that Romney is simply weak, but I think the better interpretation is that he's trying to lead a party that's become impossibly nihilistic when it comes to public policy. I mean, what non-fictional policy is Romney supposed to be specific about? Repealing the direct election of Senators?
Now, in terms of winning the election, the WSJ has it exactly wrong: the out-party is almost always best off being as vague as possible, and that's certainly the case when the incumbent party is most vulnerable (it doesn't help that their example is John Kerry, a candidate who probably did better than should have been expected). Exactly no one is going to vote against Romney because he declines to advocate specific policies, but there's always the risk that embracing one set of policy choices could alienate voters who otherwise might just want to throw the bums out.
However, conservatives are quite right to push Romney for specifics -- not because they would help him in November, but because specific commitments will tend to constrain him once he takes office. One of the key implications of the Journal's "more specifics about myths, please" editorial is how room there is within the GOP for policy-making; of course Romney isn't going to produce a health care plan, given that no Republican right now has one.
The other key implication? I've talked about this before too: normally, parties have some standard go-to issues with popular proposals they talk about to maintain the illusion of policy seriousness. The current Republicans just don't have that any more. They used to have, for example, foreign policy -- but that's just a minefield for Romney these days. Crime has faded as an issue. There's the deficit, but of course that's for vague talk only, since conservative orthodoxy won't allow Republicans to support plans that would actually reduce the deficit in any real way.
All of which is only to say, again, that Romney's substance-free campaign is a function of the current condition of the Republican Party, not anything particular about the candidate.