Audio really matters in this game. When people move they make noise. The faster they move (shift = sprint), the more noise they make. If you're defending you need to appreciate how the enemy will approach, where their various spawns are, and prepare to camp.
The less you move around, the less likely it is that the enemy will be able to pinpoint your position either aurally or visually. Moving makes you a target, and when you're defending you have the advantage of an enemy that HAS to move.
Find a good spot near or overlooking the point, in a bush or building, and wait. Peek now and then but spend most of your time listening, and looking for movement, focussing on the obvious angles of approach (use the map and use 'T' to pinpoint enemies or to orientate yourself with their spawns.
If you start taking long range fire, seek hard cover immediately. The enemy has seen you and it's only a matter of time before they land a hit. If you're in soft cover, run for hard cover. Zig-zagging helps. Again, use your stereo headphones to pinpoint where the shots are coming from and communicate this to your team.
When an enemy shoots you can tell which direction they are in, and how far. This comes with practise. Also you can tell how far they are by the time difference between bullet impact/pass and gun report. The bigger the time difference, the further away they are.
Guns sound different when fired from within buildings - more echoey and 'boomy' Learn the difference and use this to assess where enemies are located. Moving within buildings is also distinct - louder and more echoey. Use this to your advantage (and try to minimise movement when inside buildings for the same reason).
Enemy movement is the single easiest way to spot them. The more you move the more likely it is that you will be spotted - this goes for your enemy too.
Crouch low in cover and look for enemy movement. If you spot one, press 'T' to mark him and then either talk to your teammates or engage the enemy.
Wait for the enemy to stop moving before taking your shot (depending on range) unless they are a long way from cover (in an open field for example). As soon as you fire you are telling your target, and any other enemies, where you are, unless you have a suppressor, and they will run for cover. Pick your shots, make them count. Your gun gives off smoke - even with a suppressor you can be spotted. Pick your shots.
Appreciate that snipers will take the high ground on some maps and if you spot one, tag em. If they start shooting you, run for cover. Don't try to engage them - they already have you in their sights and you will probably die if you stay put and try to find them. Don't just go prone and hope - you will die unless behind hard cover. If you run into a building the enemy will tag it. Run out the back and find a good position, and expect enemies to home in on it.
Learn your enemy's uniform. After a while they will begin to stand out agains the greenery. Conversely, don't shoot friendlies! Learn what your team's uniform looks like, and if in doubt check the map before shooting (M key).
When a game starts it's important to move as fast as possible toward the enemy point, or to positions to defend your own point - always using cover. As time progresses (like after 20 seconds or so) on you need to change this to creeping in a low crouch, with long pauses while you asses the landscape and try to spot or hear enemies. You move, you expose yourself. Creep and sneak like a ninja.
Move between hard cover, and stop in bushes and behind walls. Enemies may have already seen you and will be waiting for you to stop before they engage.
As a defender, look for obvious points of ingress, tree lines, buildings, walls and soft cover. They have to take your point, you have to ambush them as they move in. Let them move - you have the advantage of being able to camp. See them, mark them, and then open fire when you have a clear shot. Thee worst thing I see in defence is players constantly running around - they die quickly. The enemy is looking for movement.
There is value on camping the point, but the best defence is to move 100-200 meters outside the point (in open areas) and pick off enemies as they approach. Read your enemy and understand how they will attack. Get behind them if possible, and move after a kill if you think your position was compromised.
As an attacker you need to find a good vantage point and scout for defensive positions. Check every bush and building, scope tree lines and walls - imagine you are defending - where would you go? Move towards the objective slowly and carefully. Stop and look/listen. Communicate with your team. Cover obvious defensive points (upper windows, tree-lines, areas overlooking the objective).
ARGO has realistic bullet drop. This means that the further away your enemy is, the higher you need to aim. Most weapons are pretty accurate up to about 200m, but beyond that you need to account for ballistics. This is hard with an iron sight, but you can see where your bullets impact and adjust accordingly. With a good scope you can use the markings/numbers and learn which mark corrosponds with a bullet impact. I tend to practise on objects to dial in my scope before engaging enemies.
Also be sure to aim ahead of running players - this becomes second nature after a while.
I will edit this as I see fit, and also based on feedback. This was a pretty quick summary and has lots of gaps. I do not pretend to have covered everything!