What's the difference between the 60, 74 and 80...

The pregroove wobble is at a higher linear frequency (cycles/cm.) on the 74 minute blanks, causing the recorder to spin the disc slower (74 min: 1.2 m/s vs. 60 min: 1.4 m/s) and thereby record more information in the same linear space. The 80 minute discs are made by decreasing the track pitch (i.e. space between tracks) from the MD standard's 1.6um to 1.5um (micrometers), the minimum allowed by the MD specification; like the 74' blanks, they also run at 1.2 m/s. There is also information encoded in the read-only TOC near the inner circumference of the disc telling the player how long the disc is. Originally it was difficult to make the 74' blanks, but nowadays the manufacturing costs of 60' and 74' discs are the same (as well as, presumably, the 80's).