Adaptation First: The Nintendo Entertainment System version of Metal Gear was the first version released in North America, as the original MSX2 version was never available in North America. Although technically a port, it has enough differences to be considered a separate game.
No Export for You: The MSX2 version was only released in Japan and certain European countries. It wasn't until its inclusion in the Subsistence edition of MGS3 that it got a wide release (particularly in America).
Old Shame: Averted. Not only are the earlier games in the series referenced in the Solid storyline, they become large parts of the story, and the original graphics are even used in flashbacks—no re-renders here. True, "Bloody Brad" and "Running Man" aren't given quite as large a remembrance as Gray Fox or Big Boss, but even so, remarkably little of the original games was retconned by later installments.
Recursive Adaptation: Played with the series as a whole. The series started out as a PC game (yes, MSX2), and in some sequels, ported to video gaming consoles, and then some of them even ported back to PC, and then a few of them consoles again.
Relationship Voice Actor: Many of the Japanese voice cast worked before together in other titles, sometimes in a very hilarious way:
Dragon Ball: Kenji Utsumi (Volgin) voiced Recoome and Shenlong, Daisuke Gori (Lt. Cunningham and Scott Dolph) voiced Mr. Satan, Chikao Ohtsuka (Big Boss) voiced Tao Pai Pai, Shōzō Iizuka (Peter Stillman) voiced Nappa, Norio Wakamoto (Gene) voiced Cell, Hiromi Tsuru (Naomi Hunter) voiced Bulma, Kōzō Shioya (Fatman) voiced Majin Boo, Banjo Ginga (King Vegeta and Giran) voiced Major Zero and Liquid Snake (and Liquid Ocelot in 4), Yukitoshi Hori (Vulcan Raven) voiced Cyborg No. 19, Toshio Furukawa (young Roy Campbell) voiced Piccolo and the late Takeshi Aono (old Roy Campbell) voiced his father, Piccolo Daimaoh. And while she didn't voice any character in any Metal Gear game, Miki Ito (Cyborg No. 18) was the voice director for the Japanese version of the original Metal Gear Solid.
The English voice acting agency got a bit lazy when it came to rehiring the original voice actors in spin-off titles. Most notably:
They're not great at continuing the actor allusions onto the English localized versions. In the Japanese version of MGS2, Olga and Solidus were played by Kyoko Terase (Meryl) and Akio Otsuka (Snake), respectively. In the English version, Olga and Solidus are played by different actors than Meryl and Snake, so the idea that both sound alike is lost. This also happened with the Bonus Boss fight against Meryl in the Substance re-release. Meryl uses the same sound clips as Olga does in her boss fight from the main game, since they shared the same Japanese voice actress, which results in Meryl gaining an inexplicable Russian accent for the fight in the English version.
Solid Snake is voiced by Peter Lurie when he guest-starred in the third Ape Escape game.
Miller's voice actors in the Japanese and English versions are different between Metal Gear Solid and Peace Walker. This is justified as the former was actually an imposter.
In MGSV, Kiefer Sutherland voiced Big Boss, replacing David Hayter.
When they first started considering a Metal Gear movie, they used an image of Michael Biehn as the model for Solid Snake on the game's box art - anticipating casting him as Snake.
Kojima had wanted to make a crossover game between Metal Gear and Rumble Roses with Rumble Roses' producer Akari Uchida, but the Metal Gear team refused to work with the Rumble Roses team. However, Reiko Hinomoto and Rowdy Reiko, her evil alter ego, would eventually appear as secret characters in Subsistence's iteration of Metal Gear Online.