For me, its the best game out there.
Theres always a populated server out there (you may not get on it immediately..)
It balances arcade and complexity/reality well.
Game lengths (30m-2hrs)are good and allow for strategy.
Maps offer variety of terrain and size (we should play a greater variety though imho).
There are many ways to play many maps in many roles.
The communication (mumble) that can and should be used in the game is brilliant. I think its what makes the game. If you get in a fun squad (or if you prefer a serious one) you can talk kaka, hang out or drive around causing havoc and generally have a real laugh (or discuss tactics).
You often have to rely on others to help you achieve your objective which encourages teamwork and communication. It also encourages community.
64 or sometimes more than 128 players all doing their thing. I love it!
The game does not only reward you for twitch reactions. I often have 10 deaths to 1 kill and feature on the scoreboard

I'm a bit rubbish and with a bad ping but I still love playing despite that. If there wasn't the communication element in the game I dont think I would have enjoyed it for so many years.
There are some players out there who are actually quite mature too...
I don't know of any game that has all these elements?
I think its a winning formula. I'd like the same format with bigger maps and more people. That is the future gaming for me. Large terrains with 1000s of people. It cant be far off...New PR!