My favourite little detail I've stumbled across in my 12 hours with the game so far was when I was driving through a dirt road with Trevor and came across a drug deal gone wrong.
There was this group of cars in a circle with their headlights on in the middle of nowhere. As I came closer, I could hear a horn sounding continuously, and spotted that the driver of one of the cars was slumped over the steering wheel dead. The dealers on both sides were either dead outside the cars, or lying wounded. I got out of the car and strolled around, putting down anyone who was lying there dying with a quick bullet to the head. Then I walked to the edge of the nearby cliff for a look around and noticed one more dealer clinging to life far below, with something glinting beside him. I walk the long way around to get down to where he fell and as I get closer I see that the glinting was a briefcase. I walk towards the guy and he begs me not to do it, but I kill him anyway and lift the briefcase. $25,000 inside. Nice.
I drive away on down the dirt road 5 minutes later, when suddenly 2 guys on dirt bikes are chasing me for the cash, and Trevor makes a comment on how he knew it was too easy.
Sadly, they caught me and killed me, but it was fucking awesome how that all came about just because I stopped unprompted.