The original Far Cry was hugely successful for two huge reasons - presentation and freedom.
When the shooter first released on PC in 2004, the lush and unrestricted jungle setting was a revelation. Gamers accustomed to dark hallways and crate-filled rooms suddenly found themselves standing in bright sunlight, surrounded by swaying trees, sparkling water and open expanses of mostly nonlinear terrain. You could stick to the path; you could leave the path. You could travel in a jeep; you could travel in a boat. And no matter what choices you made, the journey looked fantastic.
Far Cry 2 emphasizes presentation and freedom as well, but to a far more impressive degree. Take the environment. The small tropical island has been replaced by 50 square kilometers of Africa, a chunk of land similar in size to the whole of Manhattan, only covered in savannah, desert, rainforest, mountains, lakes, rivers, refugee villages, shanty towns, colonial cities, diamond mines and indigenous beasts like zebra and gazelle instead of subways and skyscrapers. You can traverse the entire swath by jeep, boat or hang glider - with no restrictions, no load times and no "levels."
Even in so much vastness, however, the small details shine. During our hands-on demo, little wisps of smoke trailed off the gun muzzle every time we fired. Our grenade's shock waves bent nearby tree trunks and knocked away their leaves using real physics. We were promised that, should we destroy a tree or raze the grass, this vegetation would slowly grow back in natural stages, not repopulate automatically. (Of course, none of this African eye candy looked as good on the 360 version we played as it did on the PC version we saw nearby. Compared to other 360 and PS3 shooters, though, Far Cry 2 looked damn nice... and is still months away from optimization.)