Ascended Fanon: Out of all the nicknames and made up names, Doomguy seems to be the one which the creators have at least accepted as a Half-Canon Name (if largely for simplicity), as it got numerous nods and actually appeared as the Marine's name in THPS3.
Fandom Life Cycle: The classic Doom games seem to be in stage 6a, with an active community.
Fan Nickname: For a long time, the protagonist of the original series never had an actual name and was referred to as "Doomguy" by the fans. It was not until the release of the novels that he was finally given a real name: Flynn Taggart.
Not Safe for Work: Doom is notably deemed Not Safe for Productivity. Its wide distribution made it one of the most popular leisure applications to be found at workplaces and universities. One quit message in Doom II parodies it.
Stock Sound Effects: The grunts and moans made by former human enemies are just some the noises made by the common dromedary camel.
Urban Legend of Zelda: Shortly after the Columbine school shooting, it was discovered that both boys were fans of Doom, which led to claims that Dylan Klebold made custom WADs based off of the school for "training". While Klebold did make levels (including a set of levels with graphics that made the game Bloodier and Gorier), none of them are modeled off of Columbine High, and when taken on their own reviewers have called the "Klebold Levels" average to sub-par.
What Could Have Been: Tom Hall, one of the lead developers, had plans to wove in a more detailed story for the game and introduce several fleshed-out characters. John Carmack was heavily against this idea, claiming that doing so would overcomplicate the game, believing that the concept of "Shooting demons on Mars" would be enough to keep the players invested. It was, but the creative differences led to Hall resigning from the company before the game was finished.
Several weapons planned for the first game included a 'Dark Claw' (which would've drained life from enemies), 'Probjectile' (same as the regular pistol except it gives you enemy stats), 'Spray Rifle' (same as the shotgun, but wider shots), a bayonet (another melee weapon, attached to the rifle that would have been the player's default weapon) and the 'Unmaker' (a weapon that would've done more damage depending on the enemy types).
Keep Circulating the Tapes: Of all the Doom games that have been ported and/or re-released on various consoles over the years, Doom 64 remains to be the only game that has yet to leave the platform it originated from. Well, not officially that is; there is a fan-made total conversion named Doom 64: Absolution that re-creates Doom 64 on PC with the advantages a PC game would offer. The project was succeeded by Doom 64 EX, a Fan Remake by the same creator which aims to be a much more faithful re-creation of the game than the original total conversion with some new added features. Second-hand copies of the game are circulating through online retailers and auction sites.
Working Title: The original name of the game was actually titled The Absolution during its development. The reason for changing the original title with the Doom 64 moniker was for brand recognition, however the working title was used as the name for MAP28. Unfortunately, this lead to many people to assume the game would be just another port of Doom rather than an entirely new chapter in the classic series.
Hey, It's That Voice!: In Resurrection of Evil one of the audio logs is spoke by Scott Menville, who did the voice of Robin in the Teen Titans cartoon. Hearing the voice of Robin speaking in utter terror about demons is pretty funny.
Knights of the Old Republic veterans may recognize Councilor Elliot Swann (played by Charles Dennis) as Davik Kang (the first game) or Lt. Dol Grenn (the second one). He also voiced Zherron in the latter game, but he's more difficult to recognize in that role.
A good majority of background characters and audio logs are voiced by SpikeSpiegel.