As an industry and genre standard, Battefield is pretty up there for open map class based multiplayer first person shooters. Years ago a free game came out that decided to take that type of game and mix it with Wolfenstein. The sequel to Wolfenstein: Enemy Territory is of course Enemy Territory: Quake Wars.
The expected standard is here in the form of basic structure. You pick a class, run around the map shooting enemies, driving vehicles, and satisfying a certain objective be it build a bridge, destroy a vehicle, or blow up a console. The unique and surprising twist is the consistency of the integration of the Quake II storyline with the base structure. The differing classes all satisfy a necessary role on the battlefied: Medic, Engineer, Field Ops, Cover Ops, and Soldier. The game does a great job of fleshing out the classes and adding unique variations if you play on Strogg or the GDF. You can build huge turrets, cannons, use iconic Quake weaponry as the Strogg, and just immerse yourself in the conflict.
The shooting is precise but not intense enough that a newcomer can't get into the action. Being only multiplayer, the game is made or broken by its activity and dedication. The level of griefing is relatively very low and I haven't seen any since playing the game months ago. The high point of activity has come and gone but what's left is a pretty dedicated and decently sized group of players.
Basically if you you've played Battlefield of some kind but want a really great take on that type of game, play this. It does enough different that it adds a lot to the genre and gaming as a whole. In a world where WWII or the modern military setting is overdone to hell, this is a refreshing break.