Battlefield 2 is one of those games you have to play to understand.
To start off, the realism of the game (when on the right settings) is remarkable, the graphics alone make the battlefield come to life with blistering tracers flying overhead, contrails filling the sky or the dust flying away as a chopper touches down. The sound is no disappointment either, the realism of firing a weapon rarely comes this close to perfection. The direction, the type of weapon and even the number of troops can be deduced simply by listening to the gunfire.
As far as gameplay goes, few games are more addictive or satisfying than BF2. First off, there are 3 factions, The USMC or United States Marine Corps, then the MEC or Middle Eastern Coalition, and finally the chinese PLA or Peoples Liberation Army. Each faction has it's unique facets stemming largely from the variation in weaponry. Though ultimately, they're similar enough so that even a novice player wont have trouble playing on an unfamiliar side.
Among the infantry, there are 7 classes: Spec-Ops, Sniper, Assault, Support, Medic, Engineer, and Anti-tank, each of which has a unique role in battle. In addition, there are many vehicles to aid these troops, jeeps, tanks, APCs, gunship helicopters, transport choppers, multirole fighters, and interceptors, all of which require a different skill level.
Also, there are different roles to play in battle, there are several squads players can make or join, each of which have a squad leader, theres also a commander of each team who rarely acts in combat, but is the single most important player in the game, as they can give orders to the squad leaders, who then give those orders to squad members. They can also use artillery, deploy supply drops and UAVs, or even drop a vehicle for the team.
The most outstanding aspect of BF2 is it's replay value. Not only are there many maps, weapons, vehicles, and alot of people to play online with, but there is a unique and extensive rank system, reflecting actual ranks in the military. Also, after a player reaches the rank of "Lance corporal" they can unlock new weapons with each promotion. The unlocks are a very nice aspect to this game, as each class has several unlockable weapons. There are even medals, badges and ribbons you can earn through your exploits.
Battlefield 2 is the kind of game you can pick up and play, and enjoy for years.
(NOTE: originally written in 2006)