Here's a quick breakdown what you'd be expected to do as each class, sorted from least to most expectations and pressure.
No special skills are required to use it to full advantage. It's your to-go class. If you're new to the game, just pick rifleman, can't go wrong with it. It doesn't have any particular bells and whistles available but it's not particularly restricted either. The equipment allows you to build a versatile attacker, possibly geared more for long-range or short-range engagements. Because it's a basic class with unlimited player slots, no one will get mad if you fail.
This class has varied selection of guns with some oddball units that allow it to be minimaxed more towards long-range or short-range battle, so it could stand in for breachers or snipers if necessary, but otherwise it plays a lot like riflemen. They're not given access to any really important gear, so like with riflemen, people don't put a lot of expectations on them. You can pick advisor over rifleman if you feel like normal assault rifles don't cut it for your battle style.
This class has access to bombs, SMGs and shotguns. This class takes departure from everyman style, because its weapon pool is geared for short range engagements. Explosives can be used to clear enemy buildings as well as to cut off their advances to your position. Strategically placed and detonated explosive can decimate enemy attack. Some amount of skill is involved in playing this class; shotguns are not noob guns, you generally only get one attempt at killing someone before they kill you back, so you better make sure that you don't miss the first time, and placing explosives ahead of enemy requires a fair bit of map knowledge.
You will be given RPG and underbarrel launcher, and you should be able to use them. The best use of this class is for taking out enemy respawn waves on their way to the objective with an RPG. But killing defenders on the objective point by precision rocket strikes into windows is good too. Insurgent demolitions guy will be in charge of taking down the helicopters, which are extremely deadly if left unchecked; this will require an RPG with good amount of target lead. Placing RPGs in locations other than enemy point requires quite a bit of map knowledge, and people will be upset if the RPG guy isn't using his RPG well.
This class is given selection of guns similar that of a rifleman, and this class has unique item "radio", making it instrumental in a proper battle. Observer plays a lot like rifleman but they must stick to a commander to take orders, and survive long enough to spell them over the radio. Their primary objective is to protect the commander, as he is defenseless while using binoculars. If you fail to follow the commander and he can't give orders, some jimmies will be rustled.
This class has access to Light Machine Guns. Despite being the most powerful hand-held guns in the game, they are not primarily used for killing. Rather, they are used for area denial. Machinegunner will ambush rushing enemy riflemen and pin down enemy snipers, to buy their team enough time to complete the objective. This requires a great deal of map knowledge, as not only gunning location must cover important paths and points, but also be out of reach of enemy snipers. People will be expecting quite a bit of effort from machinegunners.
This class has access to high power sniper rifles, which can generally even shoot through armor. But sniper rifle doesn't makes soldier a sniper - sniper makes rifle a sniper rifle. People will be expecting a lot from you, your ability to put down enemy soldiers, machineguns and snipers must be top notch. This involves huge amount of map knowledge, you must memorize every nook an cranny where the enemy might be hiding and every path they might be taking, and be there to kill them. Sniper is like a modern day ninja - fast, stealthy, deadly, with impeccable skill set - and that will be expected of you.
You will be given the power to call in fire support. Because fire support takes a while to arrive, this class requires extensive knowledge of the map as well as extensive knowledge of enemy tactic, so that the fire support location can be predicted up to half a minute ahead. In addition, simply not dying while trying to use binoculars takes a very particular set of enemy evasion skills, which will be of great importance, as the commander must expose himself to mark the fire support location. People will be very upset if you fail to call in fire support properly.