The one thing which astonishes me about BC2 is the amount of players who simply have no idea how to play the game properly. This game does not use the same gameplay as MW2, so the golden rule is DON'T PLAY IT THE SAME WAY.
How to play as a medic
The Medic's job is to keep up a line of defense or attack by healing and reviving allies. Hence the class name MEDIC. The Medic's job is not to go on the offensive with his/her M60 w/Magnum ammo and a 4X ACOG, but this is a common, sad sight.
The Medic's priority is to revive and heal. If you see a heart-monitor blip on your minimap which is close to you, go there and REVIVE WHOEVER IT IS. This will not only net you 50 or 80 points if the person revived is in your squad, it will give your team a ticket back. This is one of the best ways to earn XP and win the match.
As stated before, the Medic's priority is not to go out for kills. For that reason, don't ever use magnum ammo, marksman LMG or a sight. The only specializations you should ever use are Medic R+ and Medic H+, the Medic specific specializations. Body armor is acceptable as it can help in running to revive allies.
The LMG should only ever be used to defend yourself when moving position, defending an objective, or defending yourself on your way to revive/heal an ally. The Medic's role is not an offensive one, though if a squad of assaults is on the move into enemy territory, stick with them to heal them and revive them if necessary. Still, it is not your place as a Medic to go on the offensive. All the LMGs are perfectly competent weapons without any weapon-related specializations, and any of them can be used effectively if you practice with them. My personal choice is the PKM (for it's damage and accuracy), XM8 LMG (for the RoF and manageable burst-fire at range) and MG36 (for the default RDS and insane burst-fire accuracy). The non class-specific weapons can also be used, but sticking with the LMGs is probably best.
How to play as a recon
The Recon's job is to spot enemies for your team and support your allies from range. The class is not called "Sniper". Recon is short for Reconnaissance, which is the Recon's #1 priority. That's why there is the Recon-specific spotting scope and motion sensors available for use. The Recon's #2 priority is to take out enemies from range. That mean 4X ACOGs and Red dot sights are unacceptable. Don't EVER use them. The Recon's #3 priority is an anti-vehicle role, the C4 is more effective than the mortar strike for this, but the mortar strike can be used for more than just taking out vehicles (no, this does NOT include flushing out enemy recons).
As all the bolt-action sniper rifles are one-shot kills to the head with or without magnum ammo, DO NOT USE IT. A recon with magnum ammo and a bolt-action rifle is a bad recon. The semi-autos and VSS do benefit from magnum ammo, but learning to use guns effectively without magnum ammo is an extremely good thing, since magnum ammo can then be switched out for something more useful. If using C4, explosive upgrade and explosive marksman are a must. If using the mortar strike, then ammo upgrade and a spotting scope/body armor is a good combination overall.
Sniping is harder to do than in Modern Warfare 2, which this game is often compared to. The recon must aim above to compensate for bullet drop and in front of a moving target to compensate for bullet time. It takes a bit of practice to get this perfected so don't get upset if you don't find yourself getting kills early on. I would recommend not using the M95 unless you're a hot shot at shooting helicopters down with it. The other bolt-actions are just as powerful and have a faster fire rate. The semi-autos are bad for long range. If you do ever use them, take a slightly more offensive position and go medium-range.
USE THE MOTION SENSORS. Most recons don't know they even have them or neglect to use them. They net you motion mine assist points which can really increase your XP total if you use them properly. Same with spotting enemies. DO IT. You're a team player. Not a lone wolf.
A recon who prefers a more offensive role can use the non-class specific weapons, but still use the specializations which benefit the class as a whole, not just the weapon. Shotguns don't have a need for magnum ammo. Or the shotgun upgrade. Or slugs.
How to play as an engineer
The Engineer is a vehicle specialist. His job is to repair friendly vehicles, and take out enemy ones. As with all the other classes, it has it's priorities. The Engineer's #1 priority is to take out enemy vehicles. If a friendly tank is being attacked, take the enemy tank out. Don't go repair the friendly tank while it is under attack. It is pointless and the enemy can shell out more damage than you can repair if the enemy is using certain vehicle specializations. It will probably also get you killed. The Engineer's #2 priority is to repair friendly vehicles. The repair tool can net you tons of points if you do a good job repairing friendly vehicles. Also, getting a kill with it is possible but not recommended, as it is with the defibrillator, though lulz can ensue if an accidental kill is made. The #3 priority is to eliminate enemies stealthily with the silenced SMG the Engineer is given.
On vehicle maps, specializations which boost the effectiveness of the rocket launchers is highly recommended, as well as tracer darts. Helicopters are a bitch on Atacama Desert, pack some tracer darts and watch that bird fall out of the sky. On maps without vehicles, the Engineer can take up a similar role to the Assault, using weapon-boosting specializations, as there are no vehicles to worry about.
The SMGs given to engineers are generally very powerful at close range and rather weak at long range. It is for this reason that engineers are recommended to stay in and around buildings or other CQB areas. Some SMGs are better than others, I would recommend the 9A-91 (for it's high damage over range), the UMP-45 (good sights, low recoil and moderate damage) or the UZI (low recoil, high fire rate, good damage).
The rocket launchers can all be used against infantry and tanks, but they have different effectiveness against them. The M2 Gustav is great against infantry, but isn't that great against vehicles. The AT4 is great against vehicles, but not so great against infantry. The RPG is pretty much an intermediary of the two. Note that the AT4 works similarly to the Stationary AT, and is not guided by tracer darts. Use whichever you prefer, they can all competently take down vehicles.
How to play as an assault
The assault class' main job is to KILL. They are armed with powerful assault rifles which are built for KILLING. These are the force of the team. The assault's #1 priority is to kill. Kill everyone you see. It's your job. The assault's #2 priority is to give ammo to team mates. If an ally is low on ammo, they will flash on the minimap if they press the socialize button on you. GIVE THEM AMMO.
Specializations which work best on the assault are the weapon-boosting ones, though as with every class, learning to use the guns effectively without magnum ammo or marksman is a good idea, as it frees up the slot for a different specialization. Another good specialization to use is the grenade upgrade in addition to the explosive marksman. This makes the grenade launcher very powerful.
All the assault rifles are very powerful, and they are all fairly similar. I would recommend the AEK-971 (for the high accuracy, decent damage, and high RoF), XM8 P (for the good damage, good RoF and good accuracy) or M16 (same as the AEK-971 in effect).
How to use vehicles
Vehicles. While not present on every map, it is necessary to know how to use these machines of war properly.
If planning to use vehicles heavily, using the Engineer class is a huge advantage, but if there are plenty of engineers on your team, it is not necessary.
Tanks are the most commonly used vehicles. For a good reason, too, they are very powerful. The Abrams and T90 are better for engaging enemy vehicles and leveling buildings, while the Bradley and BMD-3 are better for attacking infantry. However, the MBTs can effectively engage infantry in a pinch, and the IFVs can take on vehicles, even MBTs, with some effectiveness. The BMD-3 AA found only on Port Valdez can easily take down helicopters, though its grenade launcher isn't very effective in taking out enemies, due to the fact it cannot be rotated 360°.
Good specializations for MBTs are vehicle alternate weapon, which gives the driver a coaxial MG which makes the MBT far more useful for taking out infantry, vehicle armor, which lets the tank withstand more of a beating and vehicle warhead upgrade, which makes the main gun quite a lot more powerful.
Good specializations for IFVs are vehicle alternate weapon again, which grants the user an AT missile launcher, which makes the IFV far more effective in taking out enemy tanks, vehicle reload, which lets the main gun reload faster for more continuous fire, and vehicle armor, which gives the IFV a greater chance at surviving attacks by enemy MBTs or engineers.
Also, once in a tank, STAY IN THE TANK, unless you are getting out to repair it. If you get out and an enemy pops in it, tough shit, expect to be killed shortly and to witness your teams ticket count decrease quicker.
Helicopters are less common than tanks, but just as deadly, if not more so. Having an enemy helicopter circling around you is indeed very intimidating, and rightly so, if the gunner catches sight of you out in the open, you're pretty much dead.
There are four types of helicopter - the attack helicopters, the Mi28 and AH64, which are armed with rocket pods and an autocannon for the passenger, the transport helicopter, the UH64, which can carry up to 5 players, and has 2 miniguns for passengers, the attack/transport helicopter, the Mi24, which can carry up to 4 players and has an autocannon for a passenger, and is an intermediary of the Mi28 and UH64, and the UAV, which can call in hellfire rocket strikes which can easily take out tanks and infantry alike.
Helicopters are not easy to fly. It requires some practice. Don't get upset if you crash your helicopter the first time you fly one, you just need to practice. Learning to do death circles and using evasive maneuvers to evade the line of fire of enemy helicopters and AA sites is a must for a good pilot. Also learning how to use the rockets accurately is a must. The gunner's job is easy, just aim and shoot.
Good specializations to use are vehicle reload, which increases the general effectiveness of each weapon, vehicle armor, which allows the helicopter to actually survive a direct rocket launcher hit (unless the firer was using explosive marksman) and vehicle warhead, which lets the weapons deal more damage.
DO NOT LET YOUR TEAM'S HELICOPTER FALL INTO ENEMY HANDS. They WILL circle around your spawn, and easily win the game. ALWAYS make sure your spawn is free of enemies who want to steal your helicopter.
There are 5 "cars" which can be used to transport your allies around the map. The quad bike is the most common vehicle, and can carry 2 players, one driver and one on the back, free to fire. The quad bike is also the weakest vehicle, and can be damaged by any sort of weapon.
There are 3 4WD lightly armored vehicles, the HMMWV, the Kobra 4WD and the Vodnik. These are all the same, and are armed with a HMG mounted as a turret with full 360° aiming. These vehicles are great for transporting allies but they will be destroyed by a direct tank hit, so be careful. These vehicles can be very useful when parked in an enemy-inhabited area and the HMG is used against the enemy. Its 360° aim is very useful, and if vehicle warhead is used, the damage is boosted.
The CAV is much more heavily armed, and again, if parked in an enemy-inhabited area it can wreak havoc on the enemy. The grenade launcher can even be used against enemy tanks, though unless they are on very low health it won't do much.
There are 2 water vehicles, the jet ski and the patrol boat.
The jet ski works the same way as the quad bike, except it is used on water only.
The patrol boat has 2 grenade launchers and can carry 4 players. It can be very effective if parked in one place and the grenade launcher is used against the enemy.
There are 4 emplacements - HMGs, Stationary ATs, AA guns and grenade launchers. The HMGs are great for suppressive fire, though they will overheat if used continuously for around 5 seconds. The Stationary ATs are arguably the best weapon to use against tanks, as it will only take 2 shots to destroy one at full health. The AA emplacements are the best weapons against helicopters, and they are also very effective against infantry. The grenade launcher is rare, but powerful when used properly. Vehicle warhead will boost the effectiveness of these weapons greatly.
Thanks to User:Zealot Guy for inspiring me to write a sort of guide, I treat his medic guide like a bible for BC2.
Feel free to comment, I don't know if anyone will be interested to read this but to those who do, I hope it helps.