Be warned that this post contains heavy spoilers for Far Cry 4.
Far Cry (and its spin-off, Crysis) has always been a series with choices. It's your choice how to approach combat, how to progress the story, and even what ending you end up with. Now that people are getting their hands on the game, they are discovering that Ubisoft have added a rather significant choice very early on in the game, which allows players to skip straight to a very clever alternate ending within 30 minutes of play time. That's about as vague as I can get without spoiling anything, so if you're cool with being spoiled, please read on.
Essentially, at the very start of the game, Far Cry 4's main antagonist Pagan Min invites the player to eat a meal with him. At one point, he gets up and leaves, and tells the player to sit and enjoy his meal for a few minutes. This is where most players will get up, leave the room, and go on to start the regular Far Cry 4 campaign. But, for the patient, obedient types, if you sit at the table for a full 15 minutes, Min will return, and the alternate ending will begin.
The video above shows the ending in full, but the TL;DR version is that Min allows the player to place his mother's ashes in their final resting place - next to her daughter, who we also discover is Min's daughter (making Min a sort of step-father to the player). You then get back into the helicopter with Min and the credits roll.
It's certainly a very creative and interesting Easter egg. I wish more games did stuff like this.