Being able to bring three local friends into an online match does a lot to enhance the overall experience. There's the obvious benefit of being able to chat and coordinate strategy in person, but there's also the added bonus of enhanced spawns: so long as one member of the party remains alive, any fallen teammates can revive quickly at a captured tower. (No need to go all the way back to base!)
While the 1.06 patch is free, the Fat Roles DLC is a premium add-on. All players will be able to see the new hat which activates the three new classes (pirates, ninjas and giants), however only those that have purchased the DLC will be able to pick up and wear the hats. Obviously, each of the classes have their own unique abilities: pirates have a gun, which can be charged for a more lethal attack; ninjas can throw shurikens; and giants can instantly kill other players (but cannot regenerate health). In addition to these base powers, the Fat Roles offer additional special powers.
On top of all this, Fat Princess has four new maps over the original game (all already-released): New Pork, Brownie Town, Candy Town and Frost Bite. The fact that Sony wanted to make this one of the marquee titles at their E3 booth is a testament to how much content they've added to the experience -- in many ways, it's a completely new game. While I have some concerns that the new characters may alter the game's balancing, it's hard to gauge with such limited preview play time. It's definitely impressive to see all that's been added to the Fat Princess universe, and fans are likely hungry to get their grubby hands on this update.