Starting with Halo 2, the complicated mythology took hold of the story in Halo games. Bungie, in my eyes, successfully reigned in the problems with that path in Halo 3: ODST, which allowed you to enjoy a Halo story without being left out because you haven't read a Halo encyclopedia, too. According to Bungie, that's going to be the approach for the upcoming Halo: Reach, too.
"Reach will continue that, because it doesn't have the burden of continuing a story like Halo 2 and Halo 3 had to carry," said creative director Marcus Lehto to Edge. "It was a true burden for us when we were making those games, because we sometimes wanted to do something but couldn't because the story wouldn't let us, or we had to support this giant steamroller of a story. Reach allowed us to start afresh."
The Halo mythology is full of interesting characters and stories, but, for example, I couldn't tell you what the hell the Gravemind from Halo 2 was. There was too much to follow, too much that required outside research to enjoy. No doubt, Halo: Reach will have plenty for the hardcore to chew on, but if Halo: Reach can make the story a little more accessible for the rest of us, I'm game.