I got to try the beta for a bit, and I can honestly say it will stomp Halo 3 into it's grave. It has a nearly Mirror's Edge-worthy physics engine (I mean not quite as good of course, but it does come close.) Like when you're looking down your sights and moving while shooting, your site's will wobble and make it hard to see, making it feel more realistic and forcing you to aim before taking your shot and not just spraying.
It does not take an ungodly amount of ammunition to kill a target, maybe 6-8 bullets in most cases (with guns having 30+ bullets to a clip usually.)
Also for both sides it has many, many character choices for you to pick from so your entire team can look different from each other for you to determine who is who (between your friends on your team) in those instances where it doesn't show their name.
It no longer has the pick up weapons around the map feature like the previous Killzone title, you choose your weapon loadout in the lobby or when you die.