To be publishable in PRL a paper must do at least one of the following: Substantially advance a particular field; open a significant new area of research; solve a critical outstanding problem and therefore pave the way for notable progress in an existing field; be of singular appeal to all physicists.
While this guideline isn't new (I kinda assumed that this is the standard that PRL had been adhering to all along), it is rather interesting that this is now clearly and explicitly emphasized. And, I must add, enforced, because I think I am an unfortunate recipient of the enforcement of this policy when one of our submission was rejected by the PRL editors.
Now, of course I'm biased since I was a coauthor, but before this, the manuscript would have been strong enough to have made it to the referees. After all, the original theory was published in PRL, and an experimental paper that partially tried to show a proof-of-principal demonstration also made it into PRL. Our paper showed not only a demonstration of a very critical aspect of the theory, but also where it deviated from our measurement. So we thought it was important enough, and certainly, important enough to make it to the PRL referees.
The rejection from the editors basically said that the content was not up to standard or not suitable. I know they are busy and inundated with tons of these stuff, but these are the times where you wish they could be specific and tell you exactly what they mean and what they were referring to rather than just some standard response. But of course, all of us listed on the paper were surprised that it didn't even make it past the editors. Usually, unless your manuscript is badly written, is clearly out of whack, or it can be seen that it is of a rather obscure topic, it will make it to the referees. But with their new policy, and also trying to lighten the burden on the referees, the editors have become a more significant gatekeepers.
So essentially, PRL is slowly becoming Nature and Science. :)
Now, don't get me wrong. It is not a criticism. I'm all for raising the standards, and the submission rate to PRL is huge. Keeping things they way they were is simply not sustainable and they will run out of referees who would be willing to perform the review. Still, I wish the editor would briefly provide a reason why, because I'm sure we could easily provide a counter argument; or maybe that is why no reason was provided.
In any case, rather than continuing on to purse this with PRL, we sent it to another publication.
Ironically, a couple of weeks after the PRL rejection, I was contacted by PRL to referee a paper! :)