People always say that "The older Halo's had a sense of Mystery and exploration, and discovery..." But what they don't realize is that when Halo CE and Halo 2 originally came out, there was no books, or novels, or Movies/Shows, all there was to understand the story was a the games themselves... Halo was new to us. In Halo CE I was thinking "Where is planet earth? What does Earth look like in the 25th century? Who are these so called Covenant? Who built this Halo ring? What do the forerunners look like? Where did the flood originate from and why are they contained on this ring? Is Master Chief a Android or a Cyborg, or a Human in a Suit? What is the Pillar of Autumn's Secret Cargo? And Why is that floating light-bulb (Guilty Spark) so surprised, and grateful to see another living person?"... At the time there was no answers for these questions, which is (In my opinion) what made Halo what it was, and made the games good, and it is what gave them there uniqueness...
The minute that these questions were answered, the novelty wore off and the games lost their uniqueness, mysteriousness, and charm. So things are better left unsaid, or Unanswered. And in Halo's case the less we know about anything, the better.
We know who/what Spartans are, we know what Earth looks like, we know who Master Chief is, we know about the flood, and the Covenant, and the forerunners and now we even know about the Promethean's... Halo can never be what it used to be, or go back to what it once was, because we know to much... The only way Halo CE is even mysterious anymore is if we erase our memories and play it for the first time, Again. (Without reading the books, or watching Halo Legends, etc first)...