Hmmm, I guess nobody really gave any thought to the reason Bungie did not "improve" the graphics very much between Halo iterations. I believe it was because they wanted to have the same "look" and feel between all three games. Imagine how many people would have been turned off from the game if Bungie had suddenly used KZ2 graphics on Halo 3. That would have changed the feel of the game drastically. Now, many of you would have rejoiced, but you are a minority. Now that you will not be fighting as Master Chief, Bungie is free to change things up a bit more.
Anyway, I am psyched about Reach, the game is gonna be EPIC.