They always say "Draw what you see, not what you think you see." There's a lot of shapes and lines that a person misses when trying to draw a face because we use faces so much - they're forms of communication and identification that become associated with everything about the person who wears that face - so its hard to step back and draw what's really there.
Practice, practice, practice - and try to use real life models, not photos or anything. Two classes of drawing naked people in college for 3 hours at a time, twice a week, is what has really helped me.
LOL I just noticed the vulture. Very nice. I wish they would of done scenes like this in the game. Some Halo Wars units would of been nice to see during the Tip of the Spear opening cutscene. I love the the way you tie all that in. 343 should take a few notes on this for the future.
It would have been very cool to have seen a Vulture fly overhead unloading a missile barrage in first person. Or to have had a Wolverine instead of the insane missile Warthog (though that's still pretty fun, too) in Tip of the Spear.