Since my first post I've been playing the game some more and thought I'd share a few more points that I forgot or didn't know at the time of writing my previous post.
1. Use your combat knife to cut through obstacles like fences and doors. The knife is used by default by clicking the mouse wheel. It can obviously also be used to kill enemies should you get close to them.
2. Always play until the end of the round, especially if your team is winning. Winning a round earns you some extra points. You can tell the situation by looking above the mini-map at the lower left corner. Above the mini-map are the tickets left for each team. Each ticket corresponds to one respawn. Medics can get them returned by resurrecting dead teammates, and in Rush games new tickets are earned when M-COM stations are destroyed. In Rush games the defenders have unlimited tickets and the attackers a finite number. If the attackers run out of tickets the defending team has won, but if the attackers manage to destroy all the M-COM stations then they will win. In Conquest games both teams start with the same amount of tickets.
3. I have recently gotten into playing as a medic. Their machine guns are nice as you mostly don't need to be afraid of running out of bullets. You can also earn a fair amount of points by throwing first aid kits to teammates that are hurt or are about to get hurt. Do also resurrect your dead teammates when playing as a medic. Get next to their body and hit them with the paddles. You can see your dead teammates in the mini-map as a kind of an EKG symbol. (Finding the body from the map might prove a bit difficult sometimes, though, as the symbol does not pinpoint the actual location too accurately.)
4. The symbols shown in the mini-map change based on the character class you are playing. As a medic you see the EKGs but as an assault soldier you see ammunition symbols when your teammates need you to drop them some ammo boxes. Do help them. Engineers see wrenches when a vehicle needs to be repaired. I don't know if recon specialists see any special symbols, but anyone driving a vehicle will see an arrow if someone wants to get a ride. When you are on foot you can indicate the need for a ride by looking directly at the vehicle driven by your teammate and hitting the Q-key.
5. Don't be afraid of trying out the hardcore multiplayer mode. The main difference is that there's no mini-map. That might not be bad, though: you certainly won't be able to see as many enemies at once, but at the same time the enemies are less likely to see you!
6. Spot those enemies. I said this in the first post already but I cannot stress that enough so I'll just repeat it here again.
7. Finally, watch this video by Pixel Enemy. The title is Battlefield: Bad Company 2 - How Not To Be A Noob: