Hey paul kelleher,
from my experiences and from what i have caught on from Panasonic and Toshiba engineers is:
on of the most commom reasons disc players stop working is the death of the diode laser.
this death can happen for many reasons:
faulty laser, a sudden jolt or the player being dropped, thus the head becomes unaligned, too much usage, etc.
remember, the laser is just that, a high intensity concentrated light. this has to be produced somewhow, so after a while, the intensity lessens, therefore the disc cannot be read or even more common, there is continuos problems reading Layer 1.
i guess it also has to do with the actual diode parts. where lower quality parts are more suceptible to failure and higher end parts go the distance.
i go through about an average of 3 players a year at my facility (across the board as far as the quality of the players) and i just change out the laser head and that seems to solve playback issues, of course, short of actual mechanical problems with the tray or the track of the tray.