A successful Letter of course begins with a valid result, one that is important and interesting. This is glib, however, because it lacks explanations of “important” and “interesting.” So, here are attempts to define each, in single sentences: An important result provides insight that changes the way others view and understand the topic, allows them to improve their own approaches, and thus leads to substantial progress. An interesting result will make readers glad to learn of it, because it is important to their own work or the work of others, or because it is science of uncommon beauty, aesthetically. In the context of a manuscript there is a third element: accessibility. Regardless of its content, a manuscript will be of lesser interest if it is impenetrable, and a manuscript that attracts fewer readers will be less important.
Present PRL policy incorporates these three concepts by seeking to publish work that should not be missed by researchers in the given field and also those in at least some related fields. Broader interest, in general, is better, as is greater importance, but the two are not independent. Work that is extremely important to a few might be as worthy as work that is moderately important to many, which again leads directly to presentation. A manuscript that can be understood only by a narrow audience will be less likely to be suitable for PRL, because it will lose its chance to be moderately important to a wide audience.