A. It'a cutscene, not a gameplay trailer. Bungie has said for a couple weeks that it was going to be part of the opening cinematic. I don't know why anyone was expecting gameplay.
B. The engine is ridiculously upgraded from the H3 engine. If you watch that trailer in HD, there are some absolutely great moments in terms of lighting and rendered detail - like when the warthog fishtails briefly as it heads into the distance. The actual warthog borders on practically photorealistic.
C. Faces are still an issue - but since everyone generally wears helmets, who cares? But the actual body animations are MUCH better. Watch the guys walk down the stairs as they leave the barracks. Watch how their bodies move. It's a far crty from the floaty/rag doll-ish animations of previous Halo games.
It's gorgeous. Now to the gameplay. Can NOT wait for the beta.