Most surprising discovery/secret/unlock/experience in this game?

[Departed]

The most surprising was the fact that there are still mechanics I'm discovering to this day, with Delta. Like how you can throw a bee hive on The Pain's platform to attract his hornets and leave him flustered, and then you can pick up that beehive after you kill him. Or the fact that you can throw snakes at Ocelot, which, for whatever reason, I never tried to do at any point until recently. The other one, I never bothered using until Delta due to how seamless camo swapping is, and that's The Sorrow's camo. It is an absolute godsend when you want to zero trace a corridor or grapple an enemy. Combing it with The End's camo during the second Ocelot Unit fight if you skipped The End makes zero tracing all of Sokrovenno so gratifying.