During the mission "Infiltrate the Crater Base", if the players choose certain truck Metal Gear Solid creator and director Hideo Kojima could be unlocked as a character to use for their team at Mother Base. Kojima has an S-rank at both Medic and Intel.
The color yellow and butterflies are prominent themes in the game because Konami wanted to have more of a pop-feel in Peace Walker that would appeal to the younger school aged kids, which were the target demographic.
The first Metal Gear Solid where all the bosses are machines instead of real people.
At the mining facility, before fighting Cocoon, you can climb one of the roofs of one of the buildings and find a haystack at the bottom. If you jump off of the building and into the haystack, you will perform a leap of faith from the Assassin's Creed series. Miller will then tell Snake about stealthy assassins using the haystack to hide from enemies.
Metal Gear Solid: Peace Walker did not start as a Hideo Kojima game. Originally, Kojima intended to lay out the overall concepts and guidelines, then let the rest of the development team take the reins. But when the themes of Peace Walker started coming together, such as nuclear deterrence and the Cold War, Kojima worried it may be too confusing and alienating for a younger audience, which was the target demographic. As a result, Kojima became more involved, serving as the director, producer, and writer, in an effort to keep the narrative as clear as possible.