Of course, an open world only works when there are things to do in it. You wear the hardy boots of one of nine playable characters, your overall objective to stop a rogue arms dealer who is engineering and profiteering from this ugly little turf war. You'll take missions from both sides, playing one off against the other so that both will help you find your quarry. At the same time, you can work with your choice of the other eight characters, taking additional jobs for them or backing them up, so that they'll return favours when you need them. For instance, helping one guy out early on means he may offer his services when you need them later. That way, when you fall in an ill-judged assault on an enemy camp, he might just pull you into cover, get you back on your feet and put a pistol in your hand. You're not fighting as a team, as such, but you'll learn the importance of building relationships with your fellow mercs as the game goes on. And the extra info these guys give you, whether in secret meetings or on your handy mobile phone (used GTA4 style to arrange missions), could help you make the most efficient use of your time and resources.
In terms of the combat style, Far Cry 2 stays true to the spirit of the original. This isn't a game of all-out blasting or super-powered heroics, it's a game where you're a lone soldier up against trained, equipped and numerous opponents, where you have to use the environment and the facilities at your disposal to get the upper hand. If, say, you have an encampment guarding a remote radio antenna that needs destroying, then going in unprepared will only get you killed. Instead, you learn to use stealth, employ tactics or make distractions. You could wire up a car with explosives then send it into the middle of the camp, or wait until nightfall (sleeping at one of the game's many safe houses) then sneak in, picking off the guards one by one. It's all about hitting hard, hitting smart and making a fast getaway. And if you leave these guys a means of transport, you can guarantee they'll be following you with everything they've got.
As in Far Cry, the enemy AI is smart, and the various groups seem to work convincingly together. At one point, I even saw one soldier grab a fallen comrade and drag him out of the way of my next rifle shot. Faced against them, you're encouraged to improvise and get inventive, but you also have other tools to help. For one thing, you can use a large scale tactical map in tandem with a spotting scope, gathering all the information you need (guard positions, vulnerable ammunition stores etc.) to plan an attack. Point the scope at an interesting area, and the data is automatically added to the map. That way, you know that hitting the building in the centre with a rocket will bring the explosive results you need to draw the guard's attention away from your real target.