This IGN article sums it up nicely. I’ve never played a game as intricate as MGS. The devs are fans of their own game, and that’s cool.
A few of my favorites (but really, all of these are amazing):
47. There is an audio tape of a soldier struggling to go to the bathroom. Play it when Big Boss is in a portable toilet, and guards won’t investigate. Is this Metal Gear Solid 5’s inevitable Johnny Sasaki cameo?
57. If the soldier comes to investigate anyway and you’ve abandoned the box, they’ll report that it was just a cardboard box. Command Post will tell them to schedule a psych evaluation.
61. Calling in your helicopter will clear the smoke from a smoke grenade.
77. There’s a way to hide knocked out enemies in plain sight without alerting his fellow soldiers – just place the unconscious soldier on a bed.
84. Scare away guards trying to investigate your area by playing the bear sounds cassette tape on your speakers.