Adaptation First: The NES version was the first version released in North America, as the MSX2 standard was never available in America. Although technically a port, it has enough differences to be considered a separate game.
The Fear's voice wasn't the first time his voice actor (Michael Bell) voiced someone from a military unit called Cobra. Neither were Volgin or The Pain.
In the original, the DARPA Chief (Greg Eagles) was Gray Fox (and Stillman), and both were also the Grim Reaper, Luke Cage and Aku Aku. In The Twin Snakes, the Chief retained his voice actor, but Gray Fox was recast. He was then voiced by the same guy who did Bubsy and Pinky.
Rob Paulsen also voiced Raphael, and Gray Fox is killed by Leonardo. Eerie coincidence!
The Jimmy Hart Version: The MGS main theme is a more heroic-sounding rendition of Georgy Sviridov's classical piece "Winter Road". The theme was first composed for VR Missions by Tappi "TAPPY" Iwase and later given a fresh coat of paint in by Harry Gregson-Williams for MGS2.
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: Proudly 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.
Portable Ops seems to play the trope completely straight however. While elements are referenced in Peace Walker and it is in official canon, Kojima more or less denies its existence, and the game was left off of a recent history of MGS poster. This is understandable given that, like Snake's Revenge, Kojima didn't direct or write it, and by many accounts the game is an inferior entry in the Metal Gear Solid franchise.
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) voices Recoom and Sheng Long, Daisuke Gōri (Lt. Cunningham and Scott Dolph) voiced Mr. Satan, Chikao Ohtsuka (Big Boss) voiced Tao Pai Pai, Shozo Iizuka (Peter Stillman) voiced Nappa, Norio Wakamoto (Gene) voiced Cell, Hiromi Tsuru (Naomi Hunter) voiced Bulma, Kozo Shioya (Fatman) voiced Majin Boo, 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 Itō (Cyborg No. 18) was the voice director in the Japanese version of the first Metal Gear Solid.
The Other Darrin: The English voice acting agency got a bit lazy when it came to rehiring the original voice actors in spin-off works. Most notably, Solid Snake is voiced by Peter Lurie (ironically the voice actor of Vulcan Raven) when he guest-starred in the third Ape Escape game.
They're also not great at continuing the Actor Allusions onto the English localised versions. In the Japanese MGS2 Olga and Solidus were played by Kyoko Terase (Meryl) and Akio Otsuka (Snake) respectively. In the English language versions 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 seiyuu, which results in Meryl gaining an inexplicable Russian accent for the fight in the English version.
As of MGS5 the role of Big Boss is now played by Kiefer Sutherland, replacing David Hayter.