Pre-1990 and post-1990 Mag-Lites are the ones with the differences. The only changes easily visible on the outside are different markings around the bezel (going from painted roll stamping to laser engraving) and the addition of a "C" or "D" to the serial number. The body tube is also a slightly different diameter.
Internally, the newer lights have a very different switch layout. I believe the newest (last couple years) Mag-Lites have yet another switch version, but there's still no obvious differences externally.
Pre-1980 Mag-Lites were a bit more squat looking, and the switch cover is much larger than later ones. A lot of these also said "PATEN PENDING" on them.