Holding off with friends in multilayer...
It was so fun to make a posse, find a cool, quiet location, gear up, and then challenge another group of people.
Another cool thing about the multilayer is that, once you picked up a weapon, you'd keep it until you left that session even if you die, so you wouldn't be restricted by rank all the time.
There's so many other features that I could mention but I'm too tired.
Overall, masterpiece of a game, spent years playing it and will probably replay it once it's backwards compatible on XB1.