I'm very excited about pretty much everything they have revealed so far. The casting is perfect, especially if you consider that they announced that they would shift ages and other characteristics around to make for a better TV show, which is always a good reason to shake things up in an adaption from one medium to another.
If it was exactly the material we already know, which was designed to be an awesome FPS, not an ongoing TV Show, the experience would suffer for it. Having something brand new that is visually informed by the source material while tying into the story we know and fleshing out characters that the games didn't get to flesh out is the way to go here.
As for the Covenant using humans, that makes a ton of sense and could actually answer something that hasn't been fully explored in the Halo lore yet. The Covenant are chasing after Forerunner relics across the galaxy but they have no way of interacting with the technology aside from haphazardly reverse engineering it based on their mechanical knowledge of the tech. They have no access to the knowledge contained within these artifacts, they can't use the tech directly despite it essentially lying around. The Covenant know since Harvest, or even before, that humans have a key in their DNA that allows them to use Forerunner tech and even in the games they haven't been above capturing humans to make them interact with Forerunner tech (Johnson in Halo 3).
Aside from that, they had to learn about humanity somehow, language, behavior etc. so it makes sense to capture humans from that perspective as well.
