My personal GOTY.
Campaign:
This is what you buy the game for. The open/free roam world will have you doing tons of things. For some comparison, the map looks a little bigger than Skyrim's. The story (I'm going to be brief about it to not give spoilers) is about Jason Brody and his friends who get captured by human traffickers. Jason Brody escapes, thanks to the help of his older brother, and now works on saving his friends. The story has twists and turns to help keep you interested. You level Jason up and get interesting perks, hunt animals for the skins to craft bigger ammo pouches and bigger weapon holsters ect. You can spend a lot of time hunting and upgrading your equipment. You can purchase a variety of guns and customize them with red dot sights, extended mags and what not. Being stealthy and taking down enemy camps with only your knife is really rewarding. There are many types of vehicles to drive such as jeeps, jet skis, cars, boats with machine gun mounts, 4 wheelers. You can even use zip lines and hang gliders. Bottom line is if you are in the mood for a free roaming, shooter, rpg then this is your game.
Co-op:
I haven't played much of this part of the game because many people are still going through the campaign. But its a 2 player offline splitscreen or 4 player online co-op campaign. It's not free roam, but still very good because of the solid gameplay the game has and you can team up with your buddies.
Online MP:
I haven't spent much time with this part either because again, people are still playing the story mode. But you have your normal game modes like TDM, Domination, ect. And a few others like firefight where you try to burn down the other team's stuff while protecting yours. You can also play on created maps that you or other people created.
Map Editor:
This is where you make maps for online MP. There are a lot of things you can do and place on your map. I have yet to spend much time with it, but I can definitely see myself spending a lot of time here once I get the hang of it.
Other noteworthy things:
1) the gameplay is very smooth.
2) stealth mechanics are good.
3) tons of side missions to do.
4) wildlife is fun to fight. Crocodiles, and sharks will surprise the hell out of you.
Overall, this game deserves the praise it gets. It lived up to the hype it got and then some. I've only gave 1 other game a 10 (Dragonquest 8) but Far Cry 3 deserves a perfect score with the amount of great things the game has to offer.