The biggest factor has been the VLSI to do the CIRC (Cross Interleaved
Reed-Solomon Code). As far as I know, we're the only ones who have
produced the 4x chips. The motors and servo stuff has been around from
"real" optical drives for a few years. But you don't to be too far in
front of the market.
Physical limits to performance? Not real limits, like diffraction limits,
etc (IMHO). But nobody would pay what it would cost. (Maybe marketing
limits are even worse than physical limits!)
Cris