**SPOILER**
Silicone:
I just checked the trailer (it took me a couple of tries) and it appears the flash of white shows something's vision of Gimli, Aragorn and Legolas (if you pause it at exactly 4 seconds in, you can see it quite clearly).
Of course, I'm assuming you mean the flash right after Legolas says "the enemy is moving". It did kinda looks like Twilight vision, but it may have just been a quick pan towards them. It does kinda look different when you watch it normally though - we'll probably have to wait until the release to see that army.
**SPOILER**
Also, in regards to the Brormir never hearing Sam's speech, he did attend the meeting at Rivendall - so you'd think the importance would be better placed in his head. I suppose though, that Tolkien was trying to show the underdog (Faramir as compared to his older brother) becoming the greater hero.
Although that may be a hard thing to do, I can't see much being better than Boromir's death at the end of FotR - other than Yoda's death, I think that's been the most impactful for me.
Finally, how sweet did that Fell Beast look swooping down onto the battle field?