It's got a more fluid movement system that reacts to the environment in intelligent ways, allowing a lot more mobility and climbing. Note that the lighter characters/classes can jump farther/higher and have more mobility, though of course less survivability.
You also have dynamically generated objectives that steer players into cooperation by offering incentives for doing the right thing. Accomplishing map objectives, switching to unused classes, etc. At any time, a player can check objectives and assist in victory, even players that typically act on their own.
There's also the character customization and outfitting that gives a sense of progression while playing. All of this is within a story that can be played offline or online with human opponents instead of AI. Overall, it's not a revolution in FPS gaming, but it's certainly looks to be a fun and exciting offering for those that enjoy class-based shooters (*raises hand*). For some, these features may seem uninteresting, but many people are really looking forward to what it's bringing to the table, hence the hype.