In short, crystals suggest two things:
1. The vanilla beans have been exposed to air for some time.
2. The beans contain the chemical vanillin.

Crystals do not demonstrate:
1. Overall quality of the vanilla bean.
2. Potency of the bean (except in the most cursory way).
3. Acceptable or pleasing flavors.

Explanation
Vanilla beans develop a frost of small crystals (called givre in French) as the vanillin inside the bean migrates to the surface while the beans dry. Givre does not develop on vacuum packed beans because there is no gas exchange in the bean. Givre thus is an indicator that the vanilla beans have been exposed to gas exchange for an extended period — recently cured beans don’t have crystals no matter how high the vanillin levels. Many vanilla beans will grow crystals if left loosely packaged.

Frost also indicates the presence of vanillin. This was an important quality indicator in the 1900s when the world was rife with vanilla bean scams. Vanillin content is the most simplistic, cursory indicator of vanilla bean quality. It does not indicate overall quality or condition of the vanilla bean, nor its flavor or “freshness”. Furthermore, great vanilla beans might not grow crystals at all:

“The absence of the crystalline coating on the vanilla beans seems to be no proof of inferiority, for Henri Lecomte affirms that it is [sometimes] absent in the best Mexican bean” (B. Sc. Pharm., 1901).

Related
Frosted vanilla bean gallery.