1. Metallica release the video for Enter Sandman (and everything that followed that in their career)
2. 1991-1992 - After the huge success of Metallica's sellout, Megadeth, Testament, Overkill, Flotsam and Jetsam, and Sacred Reich change their sound to vie for more commercial success. At least Testament, Overkill, and Flostam and Jetsam realize their mistake and correct it on the next album. (Note: I like all of those releases, except Testament, but it did piss me off how obviously they toned down their sound for commercial success, which backfired on them all).
3. Rikki Rachman on Headbanger's Ball (most notably his introduction of Sepultura's "Desperate Cry" as "Here's sepultra (missing the second u) with Desperate City". Actually, all of Headbanger's Ball was pretty much a joke. More glam was played than real metal. Should've been called "Poseurs Gay Pride Parade with some metal thrown in to get people to actually watch it".