JSON is flexible, and can store text and numeric data. It does not store numpy arrays, but rather it is limited to storing lists of data. You would have to convert them back to arrays if you want to do array math. You probably wouldn't want to store a 3d array of electron density in this format, although it probably isn't worse than a CUBE file format. We haven't tested these files very significantly yet at a large scale to see how fast it is to read from lots of them.
Nonetheless, this looks like a reasonable format to share data in human and machine readable form, without violating the VASP licence conditions.
Copyright (C) 2013 by John Kitchin. See the License for information about copying.