Warfare around the clock, massive battles like you've never seen, and a gigantic amount of content. Find a group!
Before we get into the nitty-gritty, it's important to note that Planetside 2 is completely FREE TO PLAY, using the F4P model similar (but definitely not identical) to how Battlefield F4P was executed: you can pay money, but that will not offer any real tactical advantage that a non-paying player cannot acquire simply by playing the game. In other words, money will shorten the time you need for acquiring high-end equipment, but doesn't give paying players any permanent advantage. Any player, if they put in enough time, can become as powerful or even more powerful than any paying player.
When you first log into Planetside 2 you'll be asked to choose between one of three factions in the game: the dictatorial Terran Republic, the free-spirited New Conglomerate, or the tech-worshipping Vanu Sovereignty. These three factions are constantly at war with each other, vying for control of three continents on a distant planet cut off from Earth. The war rages on for 24 hours a day, with continents switching hands almost on a daily basis. On the more popular servers, it is not uncommon to find up to a thousand (or more?) combatants, spread out between these three factions and three continents, constantly doing battle.
There are five different infantry "classes" to be chosen from (six if you include the armored suit known as the "Max"), and you can freely switch between them each time you die or "redeploy" to a different area. Careful balancing has been done to ensure that "solo" work is extremely difficult: you usually have to rely on other players, using other classes than your own, to content with whatever the enemy will throw at you. Nonetheless, in a pinch none of the five classes is underpowered: they each have their own way of fighting and can be quite effective - but are much more effective when cooperating with each other.
Vehicles too come in great variety, from tiny 4x4 ATVs to gigantic airborne transport vehicles, powerful tanks, fast interceptors and so on. Most vehicles can be manned by two or more players, whether for transport purposes or to serve as gunners for the vehicle. Transportation is extremely important because each continent is truly VAST: it can take a long time to walk from one side to another (I estimate over 15 minutes on foot, possibly more!).
With this much space to cover and so many players participating in the battle, coordination is key - and is really what distinguishes Planetside 2 from all other MMOFPS games. Command and control is an extremely important part of the game, organizing squads and platoons of up to 48 players to try and make concerted efforts to take or hold key territories. The proper mix of infantry types and vehicles is often essential, and platoon commanders spend much of their time examining the huge map to determine where best to send their troops. VOIP communications are pretty much required for commanders and also highly recommended even for the grunts, and the game offers plenty of tools for all sorts of coordination. On an even larger scale, players often join "outfits" which coordinate the actions of two or more platoons to maximize effective force - resulting in groups of hundreds of players making concentrated efforts all across the three continents. When done right, even the lowliest rifleman gets the feeling that they are part of a massive war effort being directed from on high. This is especially prevalent during "alerts", which are periodic objectives given to all three factions to fight over specific territories for extra bonuses - that's when you'll see soldiers fighting their hardest!
The degree of customization in the game is also quite staggering. Each infantry class and each vehicle can be outfitted with a huge variety of weapons and auxiliary equipment, most of which has to be purchased with in-game credits ("certifications") or actual money. "certs" are gained for pretty much any action you take: not just killing enemies, but also repairing, deploying troops with a vehicle, spotting enemies for others to kill, and so forth, are all worth "certs". You even get a few certs simply for time elapsed since the last time you played. Deciding how to spend your certs is very difficult with such a huge variety of choices, but over time you can improve your performance as any of the classes or with any of the vehicles, giving a very satisfying sense of accomplishment. As your equipment improves, the way you play the game can also radically change.
It is extremely recommended not to play this game alone. If you can't find friends to join you, do your best to find platoons and later outfits to join. Over time you can learn to lead platoons yourself, and this is almost an entirely new experience that few if any other FPS games offer. Overall, the joy of fighting alongside friends (whether from your real-life or ones you met inside the game) can't be adequately described here, it is truly rewarding.
And finally, Planetside 2 is constantly being improved, with new features and new content. Though it's still a little buggy (it is officially still in Beta, though overall it is sufficiently stable to play for hours straight most of the time) the game is technologically impressive and the development team issue enhancements, patches and fixes on almost a weekly basis. You quickly develop the expectation of logging in to find that the game has improved since the last time you played, and it is very satisfying. Plus, it's free, so give it a try and get hooked!