There is a darkly hilarious Soundtrack Dissonance when driving around in a car with the radio playing. It is entirely possible to plow through a patrol of heavily armed pirates, flip the truck trying to avoid a tree, roll several times, smash the hell out of your ride, and explode, all framed to music that sounds like it was ripped from SpongeBob SquarePants.
When you meet "the man in white":
Willis: You have ten seconds to tell me who you are before I remotely detonate the C4 under the table and this whole place explodes like a pop bottle.
Agent Willis is so bizarrely, nonsensically jingoistic it wraps around into pure comedy candy.
He also happens to be the one who writes the Database entries. Hence his own entry simply reads "That information is classified. You didn't think it would be that easy, did you?"
As another example of a database entry, the one on hang gliders:
Hang gliders are for people who look at the airplanes and think, "What's the point of that engine?" Or for people who scream as they're sucked into an engine, "Why did I go hang gliding?!"
The one about the ziplines is a good example too, especially this line:
One day, one of those is going to lead straight into a volcano, mark my words.
Some of the unexpected reactions from the enemy AI: for example, it's not unusual for an animal as small as a dog to aggro a Molotov enemy who then proceeds to throw a bottle and set half the outpost on fire.
Michael Mando and Christopher Mintz-Plasse made a series of videos showing Vaas torturing Christopher on the beach. The entire series is some of the most hilarious Vaas content you'll find, especially when he uses the firecrackers, freaks out when he thinks Chris may have died, and starts thinking of ways to revive him like a surgeon in a bad medical drama.
Vaas: Carlos! Get some water! Bring the batteries! The spiders! Everything!