What's vital to winning in most games would be, map control. Map control is when a team or person controls a key area to a map. For example, say the red team is controlling the bridge in Ice Fields. With that, they have access to some heavy weaponry (Fuel rod gun and rocket launcher), two sniper rifles nearby, and an overshield. If the other team were to have gotten their flag, then can get him if he/she is on the bridge, or under, as they can bring down vehicles with the heavy weaponry supplied, and anyone down under will get sniped at. Controlling that bridge can help win, especially when you get the flag.
Here is a list of some crucial areas you want to have under your team's control.
Sidewinder: Tunnels, Ledges. To take over the ledges, you should have a few snipers and have 1-2 people go in the enemy base upper floor and have them kill anyone who tries to get up there, or get out of the base. And you can kill anyone who tries to get the flag for a cap. As for the tunnels, have 2-3 people with pistols/shotguns/rocket launcher (maybe fuel rod) in there, and have them go on the ledge above the entrance (inside the tunnels). You can make sure no vehicles go through as well, and shotgun people in the head.
Blood Gulch: The middle section, and maybe the ledges. If you can control the middle area and the teleporters, then you can probably give them some serious hell and make things quite difficult. Also, have a person on the ledges by their base to make things difficult with the pistol and sniper rifle. That can really keep them cramped, and in a shitty position.
Hang 'Em High: The high area (Blue base, I think). You can get a great view of the map and you can get the fuel rod gun to make things quite hard for them, and is a good location for sniping/pistoling. If you can control the ledges and the slopes, then no one can get up there, really.
Danger Canyon: Tunnels, the ledges. In the middle section of the tunnel, have a person or two with a fuel rod gun and pistol take care of anyone to go for the sniper and flag. It can really prevent the other team from scoring caps. Also, have a fuel rodder outside on the ground level on that small structure in the middle to take care of any vehicles going for the flag. And a sniper on the ledge (the rock ledge with the tele, sniper, and teleporter) can really make things worse for the other team.
Death Island: The upper area. If you can get some snipers and fuel rodder/rockets by the teleporters and have them patrol the ground area from up high, then you can prevent any possibly caps easily. If you can have their teleporters guarded easily, then things will be extremely difficult and also, have a person or two to make sure they're trapped in their base or near it.
This is just some of the areas in which your team should control to have a good chance at winning. This can be critical in team games, very much.