Boy Urk is gonna be mad at me when he gets back from lunch and I'm spoiling his BWU with real-time updates.
So - Sage just specified more for me that Assassinations can only be triggered within an approx. 120 degree cone outside the players field of view (back and sides).
The assassination depicted in the trailer is misleading because you don't see the initial frames when the Spartan is getting spun around with the first hit, resulting in a face-to-face finish.
Currently in the games I've played, I recall each load-out offering a unique armor ability but almost all of them having the same weapons. Again, this is highly contextual to the gametype in question and will vary accordingly. You can totally pick up other weapons on the map though (which is why you see Jet Pack guys wielding DMRs and ARs).
(everyone please act surprised when Urk posts this on Friday)