The game itself. Has a dedicated player base, excellent combined arms gameplay, manages to balance realism and fun and is just plain cool.
Not to mention the fact that it's still going strong ten years later, and is now completely independent of BF2.
Must be played to be believed, but the community in PR is very, very dedicated to teamplay, and will help you out where they can. They also make specific squads with specific jobs and do only those jobs until the round ends. Calling Air-support and watching the enemy positions be blown up, or calling for Medevac in a hotzone while a Blackhawk spewing bullets lands in and moves your wounded ass back to base, or just a really, really good firefight with another squad in support can all be personal awesome moments.
Any well executed operation can turn out like this. From an infiltration behind enemy lines to lay mines to coordinating a simultaneous shot within the sniper squad to take out two targets before the enemy has time to react.
Or a botched operation. Becoming under fire, scramming for cover, rescuing downed teammates then managing an orderly retreat while calling in the cavalry.