Known for being the jack of all trades / master of none class, the assault kit is the definition of versatility in Bad Company 2, and that is definitely not a bad thing.
Assault Rifles - Very effective at medium and long range, and equipped with a 40mm grenade launcher. Assault rifles are second only to sniper rifles when it comes to accuracy, yet can hold their own at close range as well.
Grenade Launcher - While the grenade launcher is effective at killing enemies it's most effective when used as a tactical tool. Blow a hole in a building to gain access to objectives faster and safer than using the front door. Use the grenade launcher to take out enemies hiding in windows and small structures.
Ammo Packs - Always drop an ammo pack nearby, which allows anyone (including yourself) to resupply their ammo and grenades, and earns you extra points as well. Also allows for chain-throwing of grenades which is very effective at breaking an enemy fortification.
C4 Explosives - If the Assault player chooses a weapon besides the assault rifle (Ie. a shotgun), they'll gain access to C4 Explosives in place of the grenade launcher. When combined with an ammo pack an assault player can place up to 6 C4 explosives at a time, enough to destroy a M-Com in one shot, denying the enemy any warning or chance to diffuse a bomb. This tactic is the fastest way to take out objectives in Rush games.
Engineers excel at two things: close range combat and anything related to Vehicles.
Silenced SMG - Primarily known for having a high rate of fire and silenced, these guns excel at close range combat.
Repair Drill - Can be used to repair friendly vehicles, stealthily damage unsuspecting enemy vehicles, and cause embarrassing head-shots. Note that using the drill for an extended period of time causes it to overheat.
Rocket Launcher - When it comes to taking down vehicles, this is your best friend. Your second best friend is the tracer dart gun which allows your rockets to lock on to their target.
Anti-Vehicle Mines - Engineers can opt to swap their rocket launcher for anti-vehicle mines. A direct strike with a mine is enough to take down any vehicle in a single hit, and is particularly effective when defending against an onslaught of vehicles.
Medics are strongest at holding a position at medium to long range. They also provide help the team recover damage and can revive recently killed teammates.
Light Machine Guns - Each hold 100-200 rounds of ammunition allowing the Medic to provide deadly cover fire. These guns have a high recoil, so be sure to aim low.
Health Packs - Medics can have 1 health pack down at a time, and any damaged players within their range will gain back health. The medic can upgrade the health packs range and healing rate with special unlocks.
Defibrillators - Can be used to revive recently killed teammates or kill enemies in a rather embarrassing manor.
While the recon class appears to be a sniper, they're much much more and playing a recon class like a vanilla sniper class isn't using their full potential.
Sniper Rifles - The long range king of the game, sniper rifles pack a deadly accurate punch but have a low rate of fire and clip size.
C4 Explosives - Extremely powerful tactical explosives. Place one or more C4s wherever you want to see massive destruction, then give everyone a nice firework show as you lay waste to the map.
Mortar Strike - Calls down a series of powerful explosive mortars wherever you aim at with your laser designator.
Proximity Sensor - One of the most powerful and underused gadgets in the game. Throw one of these down and for the next few moments all enemy players will show up on your teams radar.