Well, i just wrote a novel, and then clicked outside of the box accidentally and hit backspace, so i went back a page and lost everything. Seriously i was typing for about 20 minutes.
So for the abridged version.
Riflemen are the most generic class, they are fast, and have a decent weapon that can be used well at all ranges, but excels at medium range. And seeing as how this is the most common class, they are used for capping points.
The machine gunners work best when with a team of riflemen. They really shine when they put down a lot of covering fire so that the fire team can move. Example: Karkar, point A. Also, when capturing points and laying down fire to deny entrance to the the enemy. Example: Dust2, point C.
The Sargent is the most balanced class (the US variant more so.) I feel that the Sgt is more of a support class and should hang back a little. He can use his smoke grenade to allow his riflemen to move. Also, he can switch to full auto and act as a very small SAW. Furthermore, he can put that bad boy m4 on single shot and act as a lesser marksman and pick off targets. He will have his times to rush into a room and start blasting on full auto, but his effectiveness lies in his ability to adapt and preform the duties that the rest of the fire team cannot.
The marksman is my least played, and therefore I know the least about it. When I play as the marksman, i use area denial. Also, I never use the bipod for the US sniper rifle. The ability to zoom out is invaluable, and the bipod takes that away. Also, the scope drift is little to none so there really is no point. My favorite sniper rifle would be the spring field (bolt action INS) because it has the largest scope. The dragonuv is my least favorite because the scope is small and cluttered.
The grenadier and rpg also work well with a rifleman team because they have excruciatingly slow reload times, and single shot weapons. Plus, they need to be upfront anyway, so why not tag along with some buddies? The m203 is a tricky weapon, but after spending about 30 minutes with it you should have it. remember, the "2" is a closer shot and the "1" is a further shot. (Oh, and remember to reload it when you spawn, and that it has a MINIMUM range.) The rpg is tricky because it easily nips walls and such that looked like you were clear of. The sight can be misleading on both of the weapons.
The Engineer/sapper is all about camping or room clearing. He can go solo and do some area denial. (I've had a lot of luck in the alley ways on Bahgdad.) If you're not the lonewolf type, you can be the first one in guns blazing. Example: karkar point C. Remember that the shotgun can be shot from the hip very easily, and that it's maximum effective range is suprisingly far. Sadly, even at point blank it is usually not a 1 shot 1 kill. Yesterday a guy had to shoot me 3 times point blank before i fell. If an enemy gets the jump on you, don't waste your time bringing up the sights. Start pumping into him, the kick is very small.
A note on the insurgent riflemen and militants:
The AKs are more balanced weapons, if you're unsure of what to expect, (you just joined the server) or you are on a very dense urban map, this would be the weapon of choice. Open maps such as buhriz are great for the L1 and the SKS.
General rules of thumb...
Keep your weapon set on it's highest rate of fire until you get into a position where you can pick off the targets. Example: It's a lot easier to pick off a guy from far away on full auto with the m4, than it is the shoot a guy who just jumped infront of you on single shot. Usually, the further away the enemy is, the more time you'll have to react. Basically, be prepared to be surprised.
If you pick up a fallen enemy's or teammate's weapon, reload it and adjust the rate of fire.
Communicate! For some odd reason, the game clears the text when you die or respawn, so typing isn't very effective. A mic is the best way to go, and if you don't have one, become a good listener. Your team will (hopefully) tell you where the hotspots are.
I see this one a lot, if you're taking a bunch of fire don't peak your head out. Staying suppressed is better than losing your head. Also, on maps like buhriz, if an rpk is pinning you down, stay down, the snipers will have a great target.
Some of the iron sights tend to be obnoxious and reduces your field of view dramatically, the saw most prominently. I usually aim lower so that i can see over the top of the sights and then just roll the rifle up and shoot.
The more ammo you have, the more you should be using. Sometimes it's best not to use that precious rpg on that single target that your riflemen are aiming at.
After your weapon is dry, drop it. It only slows you down. The exception would be if there is a known ammo box in your general area.