Maybe you can aim-down-sights with your hands. The player will make a little circle with his fingers and hold it up to his eye.

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1. What's ADS?

2. I liked the bloom. :)

I also enjoyed bloom, I never understood the complaints.

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The little bit of first-person view and that HUD screencap didn't seem to imply "iron sights". I think they'll stick with the classic style, like Halo and Marathon both used.

Bloom is entirely understandable as a mechanism for mitigating weapon effectiveness over range.
This is a big part of why I prefer Reach's core gunplay to Halo 4's. I would usually be in favour of the style Halo 4 aims for over the style Reach aims for. However, Reach's use of heavy bloom makes sense for the slow movement and hitscan in these games. Whereas Halo 4's minimal bloom combines with the slow movement, hitscan, and high aim assist to really mess up map movement and line-of-sight design.

That said, bloom has some issues. It leaves the player feeling like they're inexplicably waiting for the game to catch up to them, and it adds en element of randomness.

Sooooo, projectile weapons. It's really unfortunate that Halo 3's choice to combine high bullet spread with projectiles while having poor visual/audio response soured people to the idea. Keep things firing reasonably straight and in a sensible manner, and use big impact effects (Halo 1-style) to show people what the bullets are actually doing (yay gameplay system transparency), and projectiles can feel terrific. They mitigate weapon effectiveness over range in a way which does not incur lethargic gameplay, does not add randomness, and very naturally adds complexity and engagement to the gunplay by seamlessly fitting in shot-leading and prediction.