The final sequence of the first Metal Gear Solid, and with it all of the big plot revelations of the game, is initiated when Solid Snake starts using the PAL key to "deactivate" Metal Gear. From this point until the end of the final chase scene, here are the plot revelations that are made:
*Naomi Hunter's entire life history.
*The details of Liquid Snake's Xanatos Gambit.
*Master Miller was Liquid Snake the entire time.
*The existence of a bio-weapon called FoxDie.
*The fact that Snake's job was to just be a carrier for FoxDie the entire mission.
*Grey Fox's motivation for fighting Snake, and that he killed Naomi's parents.
*Most of Liquid Snake's life history, and why he looks like Snake.
*The existence of Les Enfants Terrible.
*Naomi's modification to FoxDie.
These scenes resolve the entire plot of Metal Gear Solid other than the stories of individual bosses and a few side characters. This in of itself would not necessarily be a bad thing, if it wasn't for the fact that many of these plot elements were only seriously brought up in this ending sequence. In particular, FoxDie's existence was only hinted at a few strange deaths earlier in the game. Liquid Snake's relationship with Snake was only developed earlier in the game through his strangely similar appearance to Snake, and through a few cryptic statements he makes during a boss fight. So entire complex plot threads are only seriously introduced and explained over the course of the last two hours of the game.
The result of this structuring of the plot is that between two phases of the final boss battle, the destruction of Metal Gear REX and the final one-on-one duel, the player is treated to a long monologue (maybe soliloquy?) from Liquid Snake where he explains about the entire Les Enfants Terrible project. It takes up a fair amount of time, and as a result weakens the pacing and fluidity of the final battle.
Many of these big revelations didn't even need to be put off until the end of the game. Unlike the information on FoxDie and such, which would have derailed the plot if Snake learned about it earlier in the game, much of the backstory of Liquid Snake could have been told earlier in the game. Knowing that Liquid Snake was Snake's twin brother, or that they were clones of Big Boss, probably wouldn't have seriously changed Solid Snake's actions in the game. Ditto for much of the information about Grey Fox. In fact, revealing more information on Liquid Snake earlier in the game could have made him more compelling of a villain earlier in the game, since he spends most of the story a complete enigma.