I'm playing FC2 right now... And it's really REALLY fun. Basically it seems completely unlike Far Cry 1 in every manner other than there are guns and the name is similar.
The gameplay seems to be a mix of... Er... Actually I'd say it's almost the same as Mercenaries. You fight for factions, gain rep (though at the point I'm at, I'm not sure what rep does exactly), steal weapons and vehicles, and go on missions.
But even still, the fire elements are amazing. When you can get a flamethrower and keep enemies at bay because the fire in the grass is dynamic and spreads... Then you realize how far we've come.
Despite the similarities between it and Mercenaries... I love it. I loved Mercenaries, and I love the African setting. I haven't tried the multiplayer, but I certainly will.
Comparing it to CoD4... Well, CoD4 IS a great game, no argument there. However, I would say single player-wise, if you like GTA or Oblivion or Mercenaries kinda open-ended worlds, then FC2 is better.