Not mandatory, but is said to add a lot to the experience.
According to a preview in Gamepro, Killzone 3 is very much in development and promises a host of new features over its predecessor.
The headline grabbing addition is 3D, suggesting Killzone 3 will form part of Sony's push into the 3D marketplace. As expected you'll need both a 3D TV and glasses, meaning the functionality will be optional, but according to the mag, very worthwhile.
If you're not 3D-enabled then there's still plenty to get excited about. Bungie's Halo: Reach isn't the only upcoming FPS to feature jet packs. Guerrilla's Killzone 3 will sport strap-on flying machines too.
There's loads more over on the GameSpot forums.