Before you angrily hit the "Back" button on your browser, hear me out. In the previous Halo games, the Covenant were like movie villains; they could even quip one-liners at you. Some would even squeal, comically, as they ran away from an overpowered Master Chief. However, because Halo Reach isn't a story about a triumphant victory -- rather, a tragedy against an overwhelming alien force -- Bungie was left with a challenge: how do you reinvent the Covenant -- familiar to a generation of Halo gamers -- and make them threatening again? You make them truly alien.
While it may seem like a rather cosmetic change, this creative decision represents a maturity in Bungie's storytelling abilities. The developer wants you to take the story seriously -- so much so that the flaming helmet included in the $150 Legendary Edition can't be used in the campaign. (Apparently, having someone's head on fire in cutscenes drastically reduces its gravitas.)