Each Class has things that they are strong in. Thing is, most people have been playing mw2 (nothing against it) and that has a completely different play style. Here are some Tips depending on the class your playing.

Assault: I don't have much for assault, as i personally hate it, but it is useful.
1. Something useful about assault is the grenade launcher. Use it to blow open buildings for easier entrance to a rush target, or to destroy cover being used by snipers and enemy team members. Players who can't adapt to the change will be easy fodder as they move from one place to the next. Also, bonus points for killing an enemy with a building.
2. Ammo Boxes, Use them. Yeah they refill your ammo so you can be rambo and go gung ho on everyone, but you get points for other people using them! Throw them down everywhere, around Snipers (people playing recon, who don't know how to play recon) and just randomly around your base if your defending. But most of all, do it a lot! The more boxes, the more likely you are to get points.

Engineer: Engineers are tactile, and can be used as assault, and can take out vehicles.
1. If you don't use your pistol, use the tracer! Tracer everything, if you are at an enemy base, why not tracer that tank? when someone gets in, everyone knows. It also nets you bonus points, but don't just trace, Tag it too if people are in it. When it is destroyed, Profit
2.If you are less gung ho, and are defending on a map in which their are currently no helicopters, Why not try out the tank mine. Position it on a downward sloping hill to make it harder to see. Or if you are feeling like a martyr, run towards it as it drives towards you and throw it down. If you're lucky a medic will revive you, in which you both benefit from the points.
3. You have a repair tool. Use it. If their is a wrench on your mini map, they need you. It takes like ten seconds, and depending on how weak it is. You can get 50-100 points.
4. Jump in a heli and repair it. Heli's can help control the battlefield, and getting it shot down is bad for business. Keep it alive and not only do you get repair points, but when anyone gets a kill. You get squad passenger points. Profit

Medic:
1. Heal > Revive. Heal them before they are dead. If they have like 1% health. You can get around a hundred points, 200 if they are a squadmate. Compare that to your flat fifty or 80 you get for revives and you can easily see why most ace players are medics.
2. However, Revive everyone possible! The defib unit is a one shot kill, one shot ... bring back to life. you get fifty points (or 80) per revive, and if attacking, your respawn ticket back. Although not as many points as a medkit, it keeps everyone on the front lines. And if they die again, Hey more points
3. Stay close to team players, but don't necessarily shoot your gun. Being a medic is more about saving lives than taking them. Shooting can give away your position and make you vulnerable to death, which is bad because you don't get points for dying. however
4. When playing in a team, if your guy was mowed down by someone else, kill the enemy before reviving your teammate. Sounds simple, but alot of medics attempt to revive before taking care of the problem in the first place.
5.Med kits: Use them. Same as assault. Throw them everywhere as often as possible. Your points will build up quickly and your team will stay healthy.

Recon:
1. Radar Balls, Throw them. Motion mine assists are an easy twenty points. Throw them into firefights to net easy points and throw them randomly and hope to get lucky with enemies coming that way. Throw them into a rush target so you can see when it is being diffused. Find Assault, refill, and throw some more.
2. Tag randomly. Chances are you aren't going to one shot someone with the sniper unless you have only played sniper and are that good. So tag everything. Not only does the enemy now have a red dot above them, but if you shoot them one, and someone else kills them. You still get 50 points as if you killed them. With other bonus's to.
3. C4> Mortar. You are Recon, not sniper. Point out enemies, destroy buildings and tanks. Throw a radar ball, If you see a tank, hide behind a wall, throw a C4 on the side, and detonate. Easy points, and no more tank. The mortar is slow, however if you must use it. Use it often! On buildings of defenders, At the attackers Base. everywhere always.
4. Moving > Camping. If you kill someone. Move. Even if it is to the other side of wherever. There is a killcam. They may not know your exact location, but i'm sure they are going to try and find you. So move and kill them again.

General:
1. Groups > Solo. Groups get you more points and increase your chance of survival. No, you are not a lone wolf.
2. Tank parties. 4 people in a tank is a useful tactic. You can cover all directions and normally kill engineers and recon before they have a chance to blow you up. Mortars are not a problem if you keep moving.
3. Use the parachute. Run up buildings and parachute down to other places, and jumping out of a helicopter and parachuting behind lines is great for gurriella warfare.
4. Find alternate rounds and Blindside the enemy. Don't run head on like an idiot
5. Use C4 as traps. If you can't get behind or to the side of a tank. Throw it on the road and hope they drive over it (don't forget to detonate)
6. Use equipment that recharges as much as possible! You have unlimited of it, why wouldn't you.
7. Plant C4 on objectives (rush or conquest) and detonate them. on Rush it gets you some points, and kills if people are disarming (capturing).
8. Stay together, but don't bunch up. Working in a team is good, but if you're to close a lucky grenade could ruin your party.
9. Destroyed vehicles are cover. Who would expect an attack on the other side of a tank that is blown up and still on fire. Not them.
10. TAG EVERYTHING

P.S. I put this under "About destructoid" because their is not a multiplatform post tag that i'm aware of. Hope these help.