Metal Gear Solid wasn't the first game based around stealth, but it was the one that set the standard for future stealth games to match up to. Despite the sea of imitators that followed, the Metal Gear series has managed to retain its own unique identity, with its iconic characters and set-pieces. These set-pieces would often be repeated and referenced in its sequels, sometimes in ways that figured directly into the plot, such as in Metal Gear Solid 2: Sons of Liberty and its mind-bending plot twists.
Then there's the close relationship that the first Solid game had with its MSX2 predecessor, Metal Gear 2: Solid Snake. While it obviously references the events of Snake's previous mission in Zanzibar Land, many of the plot beats and gameplay concepts are carried over or re-used. Metal Gear Solid took the ideas present in Metal Gear 2 and, free from the MSX2's hardware limitations, refined them into a more dynamic, cinematic experience.
9 An Old Ally Returns as a Ninja
Gray Fox, Snake's friend-turned-foe from the previous games, reappears in Metal Gear Solid as a cybernetically-enhanced ninja, hell-bent on one final battle to the death. He wasn't the last ninja to pop up in the series, as Metal Gear Solid 2 and 4 would feature similar characters, nor was he the first; that award goes to Kyle Schneider, aka Black Ninja.
Schneider, part of a resistance group within Outer Heaven, aided Snake over the radio in the very first Metal Gear. Presumed dead after being discovered by the enemy, he resurfaced in Zanzibar Land and fought Snake as Black Ninja, powered up by experimental drugs and armor. He died by Snake's hand, but not before making amends by giving Snake one last hint for his mission.
8 Snake's Number One Fan
In the snow field between the tank hangar and the warhead storage facility on Shadow Moses, Snake received an anonymous call over his Codec. The caller warned Snake that the snow field ahead was littered with Claymore mines, and he would need a mine detector to find them. He told Snake to simply call him "Deep Throat" but, after being pressed further, the caller eventually divulges that he is "one of (his) fans."
"Deep Throat" turns out to be Gray Fox, although Snake ought to have figured that out, since Fox pulled the same trick in Zanzibar Land. At that time, Snake was crossing a desert and was once again interrupted by an anonymous tipster warning him about a minefield up ahead. Fox didn't use any specific pseudonym that time, simply describing himself as Snake's "number one fan."
7 A Female Contact In Disguise
In both games, Snake needed to find someone who had disguised themselves as one of the enemy soldiers. In Metal Gear Solid, it was Meryl Silverburgh, who accompanied Snake until she was injured by Sniper Wolf. Her fate depended on the player's ending, but her appearances in later games confirms her survival. The same can't be said of her Metal Gear 2 counterpart, Gustava Heffner.
Like Meryl, Gustava (called Natasha Marcova in the original MSX2 release) was undercover and was found in the women's bathroom. Later, an ambush on a bridge has her being fatally wounded in the crossfire. Snake's love interest Holly White would also disguise herself as an enemy soldier, while managing to survive until the end of the mission.
6 Elevator Ambush
Once Snake hopped into the Communication Tower elevator, after it initially failed to come online, he got a call from Otacon warning him that his spare stealth camouflage devices were missing. Tipped off by the elevator's weight limit going off, Snake was attacked from all sides by invisible guards who had been waiting for him.
This encounter was presumably inspired by the Four Horsemen boss from the previous game. Snake was likewise surrounded inside a cramped elevator, although the Horsemen didn't have stealth camo (or horses, for that matter) at their disposal. Instead, the assassination squad hid up in the ceiling and periodically hopped down to take potshots at Snake.
5 Shape Memory Alloy
Meryl and Gustava share another link, in that they both provided Snake with a key that was pivotal to advancing in the game. Meryl gave him the PAL key, which could deactivate Metal Gear REX's nuclear warhead. It turned out the card was actually three keys in one, being made of material that changed its shape when exposed to certain temperatures.
Gustava gave Snake her brooch, fashioned into the shape of Zanzibar Land's symbol. It was also made from a shape memory alloy, turning into one of two keys when cooled in a freezer or heated in a sauna. When heated, it let Snake open Gustava's locker and get a tape of Zanzibar Land's national anthem, which he could use to make enemy soldiers temporarily stand to attention; when cooled, it unlocked the locker containing the mission-critical OILIX formula.
4 Spiral Staircase Chase
After reaching the Communications Tower on Shadow Moses, Snake was spotted by an inescapable security camera, forcing him to flee up the tower's staircase with the enemy in pursuit. Once he'd evaded the soldiers and reached the top, he was ambushed by Liquid Snake in a Hind D. His only means of escape was a rope he'd picked up along the way, which he used to rappel down the side of the building.
A very similar chain of events occurred back in Metal Gear 2, where Snake had to ascend the Tower Building via the stairs after its elevator malfunctioned. He unavoidably ran into some guards who endlessly chased him all the way to the top floor. Once again he had to make a daring escape, this time with a hang-glider from the building's highest balcony.
3 Check the Back of the Box
When Snake tried to get Meryl's codec frequency from Kenneth Baker, the ArmsTech president admitted that he had forgotten it, but that Snake could find it "on the back of the CD case." This was one of the many times that Metal Gear Solid broke the fourth wall, as Baker wasn't referring to any in-game object, but the case for the game itself. A screenshot showing Meryl's number is featured on it.
Meanwhile, back in Metal Gear 2, Colonel Campbell regularly changes his radio frequency in order to avoid their communications being traced. Most of the time he will simply tell Snake what the new frequency is, but on the final change he gives similar instructions as Baker. Sure enough, a screenshot can be found with the number on the back of the game's packaging.
2 Hind D, Meet Stinger Missile
Snake confronted Liquid at the top of the Communications Tower in a snowstorm, armed with a Stinger missile launcher while the latter piloted a Hind D helicopter. After a grueling battle, Snake was able to shoot down the Hind. Otacon may have been impressed by the feat, but Snake considered it to be no big deal. Perhaps it was because he already had some practice...
While a Hind battle did occur in the original Metal Gear (except in the NES version), the fight took place entirely on the landing pad, and it was destroyed using a grenade launcher. Metal Gear 2 didn't give Snake that luxury, as he had to take out an airborne Hind using Stinger missiles. Compared to the beating that Liquid's took to go down, it only took four of Snake's missiles to turn this copter into scrap.
1 A Final Fist Fight in a Dangerous Arena
The destruction of REX at the end of Metal Gear Solid was only the penultimate boss fight. What followed was a hand-to-hand battle between Snake and Liquid atop REX's head. Snake had to be careful not to stray too close to the edge of this makeshift arena, while trying to knock Liquid off once his health was depleted.
In Metal Gear 2, Snake had to battle Gray Fox after destroying Metal Gear D and losing his equipment in the resulting fire. Fox led Snake into a small room where he had lined the perimeter with mines. Snake eventually managed to prevail, but it wasn't the end; he still had to confront Big Boss himself.