Some grenades are not cooked, and will lounge around for a few seconds after landing in front of you. Others are fully cooked and explode immediately. The rest are somewhere in between.
Press F to pick up a grenade, which triggers an animation to throw it. Position yourself so that once you pick up the nade you'll throw it at the enemy, or at least away from your friends.
You must be swift! Sometimes you'll die anyway if the nade was cooked too long.
Either lob it underhanded or throw it overhead. If you right-click for a short underhanded toss, then left-click to cook. If you left-click for a long overhanded throw, then right-click to cook.
Kicking Down Doors
Kick a door open with the middle mouse button.
Stay away from doors unless you plan to open them or kick them down. Because if a door is kicked open and you're on the receiving end, it will kill you.
Downing a Helicopter
Helicopters are only called by Security forces.
Playing as the Insurgent class, you'll find yourself at the business end of some seriously tough helicopters.
Playing as Security though, those helicopters hardly ever earn their keep! The occasional 8-10 kill streak happens, but sometimes they die from the godlike precision of a bot that shoots 'em down 4 seconds after appearing. :)
Your best chance of taking down a heli as an Insurgent is by hitting it with an RPG. Your other chance is by shooting its rotors with a machine gun. It takes a lot of shots, but it's possible to down a heli with a machine gun simply by targeting its rotors.
Commander + Observer
The Security Commander uses the O key (O by default, it's configurable) to call in A10 gun runs, miniguns, explosive ordinance, smoke mortars and a badass helicopter run with an initial rocket strike + 30mm cannons.
The Insurgent Commander also uses the O key to call in drone strikes, smoke mortars, chemical mortars (gas mask may be required), and more.
The Observer and Commander have to keep close enough to each other so that the Commander can tell the Observer the target location. The reality is that the Commander may not call in squat, or blow his entire load on the first two points of the match. Inexperienced Commanders often call in the wrong solution to the wrong target at the wrong time, killing teammates in the process.
Having said that, a good Commander + Observer duo can turn the tide of war and lead the charge to victory.
Call in a chopper or A-10 strafe at the start of a counterattack to mow down the enemy as they run towards your position.
Avoid calling them on objectives where the enemy are holed up...or you'll get a few kills, maybe. It's a lot of noise and fury for a tiny ROI.
Destroy Ammo Caches
Some points must be destroyed instead of capped. Either stand over the cache and hold F to slowly attach a makeshift IED, or let someone else destroy it faster with an incendiary, molotov, C4 or grenades.
It takes 2 grenades, or 1 of the other devices, to destroy a cache.
Beware the Drones
Insurgent Commanders will call drones to fly into the building you think you're safe in. Try to shoot down drones. YMMV. Good luck!
Smoke mortars are fantastic when used correctly. My subjective, but repeated, observation is that the closer you are to the edge of a smoke cloud, the more likely an enemy is to detect you.
A10 strafes always come in right to left from the perspective they were called in. Gunship rockets fire in the direction you're looking when calling them in.
Callsign Cleric is the helicopter with miniguns only (no rockets). Callsign Assassin is the chopper with rockets. Callsign Warlock is the A-10.