Spoiler:
For beating the game, you unlock a couple of mini games - Assignment: Ada, and The Mercenaries...yeah, THAT Mercenaries game from RE3. Plus, doing well in the minigames unlocks quite a few extra weapons. I already got a Tommygun with infinite ammo by beating Assignment Ada. Nice...

Oh, and there's Professional difficulty now, and the ability to play through the game again while keeping all your custom weapons.

Spoiler:
The lead infection behind all the monsters is a parasitic thing called Las Plagas. It was unearthed by a cult called Los Illuminados, with the assistance of a rich midget guy named Ramon Salaazar, whose ancestors supposedly sealed Las Plagas away underground hundreds of years ago. The Las Plagas were dormant in a fossil-like state, and spread themselves like spores once they were dug up by the local villagers. They began infecting the villagers, which spread from there. The Plagas soon grew into thier full adult state in thier new hosts.

From what I gather, the leader of Los Illuminados, Lord Saddler, became the "Overmind" of sorts for the Las Plagas, being able to control anyone or anything infected with a Plaga. Hence, the infected villagers (aka "Not Zombies") were dubbed Ganado (meaning Cattle, or something like that).
From there, Saddler experimented on the Las Plagas. One of the researchers, Luis Sera, bailed out after the experimentation went too far, and is being hunted by the Los Illuminados.

Saddler's ultimate goal was to conquer the United States of America, and use it's power to infect the world. His strategy? Kidnap the President's daughter, Ashley, infect her with a Plaga, and ransom her back. Once she returns home, she's fully infected, and goes nuts, supposedly killing the President. Los Illuminados steps in in the confusion, and quietly infect the US with Las Plagas. (Something like that)

Meanwhile, a mercenary, Jack Krauser, is the one to actually kidnap Ashley. He did it to gain Saddler's trust so he could get a sample Plaga specimen. Why? Because he is working with none other than....Albert Wesker. Wesker also sent another agent to get a Plaga sample, one who would be working outside of the Los Illuminados knowledge: Ada Wong. Wesker's ultimate goal, from the looks of things, is to get Umbrella back up and running again. However, it seems like Krauser and Miss Wong both have very different ways of getting the job done...

Then, there is our friend Leon. I won't give too much away about what happens to him, as he is pretty much the glue that bonds all this together, and I think I've spoiled enough here already.

Spoiler:
Battle System?