The first and most important difference is each game's population. I am not exaggerating when I say that the original PlanetSide is today merely a shell of its former self. I tried on several occasions to find a fight to join and was only once able to locate a tiny battle in one small corner of the world. That's something to keep in mind if you give the elder game a deeper look.
PlanetSide 2 uses the SOE-created Forgelight Engine, allowing for more realistic vehicle controls, day and night cycles, bullets that are affected by physics and other bells and whistles. It also introduced locational damage to the mix, meaning that a well-placed head shot can actually cause more damage than other shots, for example.
The more obscure differences between the two games are relatively obvious to veteran players. Battle Frame Robotics aka powerful walking mech-suits of death aka "Big F!ing Robots" were an original-game addition that some players have referred to as "PlanetSide's NGE." As with Star Wars Galaxies' NGE, there are still debates as to how much of an impact the changes truly made, but some players claim to have left the game forever simply because of the addition of the initially overpowered behemoths. Similarly, forum-goers insist that if BFRs (or something similar) were added to PlanetSide 2, there would be boycotts and significant population loss. However much BFRs imbalanced the classic game is still up for debate, but either way, BFRs have become attached to the PlanetSide story.
Fortunately, SOE president John Smedley has promised that no BFRs will appear in PlanetSide 2. Balance is an ongoing issue for any massive PvP MMO, and new vehicles like the Harasser are introduced frequently, bringing the possibility of a new BFR with them. Time will tell.
How players pay for both games shows not only a difference in the titles but a difference in how modern MMO players favor free-to-play. The original PlanetSide was a subscription-based MMO, and players had access to everything the game offered through gameplay. But PlanetSide 2 uses a not-so-subtle item-shop that sells many things you need to become a better player. With enough money, a new player could potentially unlock almost everything in PlanetSide 2 within minutes of loading the game for the first time. By contrast, the original title did not sell access to better weapons and skills. Instead, those items were unlocked through gameplay.
Many players see PS2's method as "selling power," but the advantage of a skill-based game is that time and experience is always the greatest balancing force, well above the influence of a player's pocketbook. In other words, although a new player can buy access to a better weapon, it's very possible that a different player who has more experience or who might even be outfitted in lower-grade equipment could best the paying player through skill alone. PlanetSide 2 does offer a membership option that gives out more frequent and better chunks of free certifications (used for unlocking weapons and abilities) as well as experience and resource boosts (used for obtaining vehicles, equipment and aircraft while in game). How effective a player is with these bonuses is still up to the individual's skill level, however.
The two titles also differ in how players craft their characters over time. In the original, a player would make a new character and log in to the game. The character would be pretty basic, and accessing a terminal would show the abilities and equipment that a player would need to unlock by acquiring certificates.
Whatever the differences, no one can deny that PlanetSide 2 is a definite step into the "next gen" world of MMOs. How long will it last before it is updated to the next tier, and how will upcoming additions like Battle Islands or the influence of Major League Gaming affect development? Those battles will be determined by the future.