The world itself. There ain't much besides the jackal audio tapes and diamonds but the world is so well built that it makes you want to traverse and see every little hill, waterfall, hut etc. The emptiness gives you a sense of solitude that you don't get with other Far Cry games and it is also closer to 'reality' because there ain't much to do or see in a savanna except for the savanna itself.
Such things never spark my interest. I need something to do in a game. Just getting on the top of a hill and then looking around in all the empty just isn't enough for me. Maybe it's because I'm doing too much world building, etc. by myself, so I'll see empty space for what it is, no matter the pretty graphics. There might be a nice view, but I'll turn around in 10 seconds anyway.