Looked at the first 100 of my HD DVD's and, it is clear that the older the disc the more likely that it will either be unplayable or have bad sectors.
The "ROT" presents in 2 forms. Either areas of speckling on the disk, or as an amorphous areas of opacity on the disc.
Under the microscope these areas in the body of the disc itself and not on the surface.
The Jewel case adjacent to the reading side of the disc may be greasy and have multiple circular impressions around the outer perimeter of the disc, and resembles a smaller version of the disc itself!
Incidentally, almost all these discs are stamped Warner Bros., and range from 2001 through 2007, however, this i not surprising since WB is the largest manufacturer of discs.
I have also started to see degeneration on the older BRD's in the same form and fashion.