An unforgiving player-run world that you will find either dull and frustrating, or immersive and exciting.
The game is not friendly. You'll get killed for no reason whatsoever by other players if you roam around by yourself. There are no PVP restrictions built into the game system - if a player sees you, he can potentially kill you on a whim.
There is little structure to the game system. You receive random objectives automatically (the closest things to quests) that you may complete for small amounts of exp and credits. These objectives are dull and have no effect on the game world. Join a group to complete objectives more quickly.
The main content, and the charm, of the game comes from the interactions of the players themselves. When you join a faction, you really join: their battles become your battles, and players cooperate to achieve the faction's own agendas. You'll follow a command structure, follow higher-ranked players into the heat of battle, and kill your faction's enemies in an organized faction. A high-ranking member of your faction may orchestrate an attack, or you may at times be called upon in faction chat for assistance against a raid. This kind of cooperation, in a game that does not enforce cooperation at all, is truly impressive.
Faction leaders make rules that you are expected to follow (uniforms, which factions are to be regarded as enemies/allies, etc). If you do not like following instructions of other players as a low-ranked player, you won't like this game.
The combat itself isn't too exciting; shooting lacks intensity and firefights consist of players running in circles shooting at other circle-running players and using health packs/healing implants.
When there is no inter-faction political tension or conflict, that is when the game's excitement hits a brick wall. When two factions aren't trying to kill each other, the game can suddenly feel extremely hollow. Economic objectives can take over your attention from there, but if that isn't your play style, there's nothing left in the game but player-killing and mundane objectives.