Its greatness can never be expressed with the written language.
The action is fast enough to get you into a battle, but slow enough where you're not going to get one-shot killed (although a sniper rifle will take you out in two shots).
The variety of jobs to pursue - sniper, tank driver, buggy gunner, fighter pilot, bomber pilot, etc. - is so large that I've been playing for six years and I still haven't done everything.
This is the one of the only MMOs where your abilities are 100% player skill. New levels only give you access to different weapons, not neccessarily better weapons. So a great fps player at level one can easily destroy a level 20 character.