An honest question: As someone who doesn't own an Xbox 360 and who has never owned a Halo title past the first one, could someone explain why this game is such a big deal?

I mean, ODST was released just last year. When they announced ODST/Reach, I thought they were basically two really similar/identical games.

Are people just obsessed with all things Halo? Is Halo like "New Restaurant Syndrome" where people pitch out the old and go to the new as soon as it's out? Or is there (most likely) something I'm missing here?