Speed is increased because the data is read out in parallel, not in series as with existing compact discs.
Data density is improved because the information is coded to make best use of available space.
There are limits on how much data you can store in a compact disc. For blue light, this spot size is about a quarter of that for a red laser, so many researchers have been concentrating on finding cheap, blue lasers to fit in their systems. Another issue is that of resolution. If two bits of value 1 are too close to each other, then a zero in between them might not be resolved. The separation distance between two bits of data depends on the shape of the waveform used and is decided by the Raleigh criterion.