Bungie looks like they really want their last Halo game to go out with a bang. They've really delivered in what seems to be worthy successor to Halo 3's content packed Limited and Legendary editions.
A detailed breakdown is laid out below.

Limited Edition:

In addition to the game disc and manual, the Limited edition includes:
  • Game disc housed in recovered ONI ”black box”
  • An exclusive in-game Elite armor set for use in multiplayer modes
  • Artifact bag containing Dr. Halsey’s personal journal and other classified documents and effects that unravel long held secrets from the Halo Universe
Legendary Edition:

The complete Halo: Reach collection. In addition to the game disc, manual, and the complete contents also found in the Limited edition, the Legendary edition includes:
  • Noble Team statue expertly crafted by the artisans at McFarlane Toys. Individually molded, hand-painted and individually numbered, this statuette is a must have for any serious Halo fan
  • UNSC-themed custom packaging
  • An exclusive multiplayer Spartan armor effect
Price:
The Standard edition will be shipping at an estimated $59.99 and the Limited and Legendary at $79.99 and $149.99, respectively.

My Thoughts:
To be really honest, I'm not that much of a fan of the Halo storyline or universe which makes the statue and journal kind of useless. The boxes seem cool also but not really worth it. The only real reason that I'm even considering buying the Legendary edition is due to Spartan armor effect and the Elite armor set. All in all, I love Halo's gameplay so I will probably will end up buying it.

If you want to buy either the Limited or Legendary editions, better pre-order them fast as they are only available while stocks last.