That's what happens when a former Art guy is leading the game. Lehto is awesome, but someone else should have had his job for Reach.
And imagine if Guardian was cut. Goddamn...
Well, you have to see what happened before:
Halo 1: Multiplayer levels were done by two guys figuring out the 'extrude' tool two months away from release
Halo 2: More art contribution, but the disc maps have obvious asset-reuse, and not very detailed. The DLC looked awesome, though, since they were able to put their full artist team on them. Even the lighting and skyboxes in the later DLC were better.
Halo 3: Disc maps look nice, DLC looks great. So just like 2, it was obvious that the DLC maps were 'later'.
To me, Reach feels like the disc maps are on par with later DLC. The downside to the multi-artist-drifting process though, is some more experimental or not-as-strong-but-loveable maps might have been left on the cutting room floor, as they had to be MUCH stronger to justify the artists' time. We wouldn't have gotten something like Construct from a Reach-style map design process.
Although it's funny now to watch the ODST panel at PAX before it came out, and they're talking about how the Mythic maps appeared as part of ODST's campaign at one point in early designs, with the only vestige of that being Orbital in the bonus ending.
ps if I were to guess, Not Boneyard was either in the destroyed Alexandria location that was cut, or perhaps was an Invasion map that used the boats that Lehto wanted to get working. If you had boats and Invasion, what developer WOULDN'T have a beach landing scenario?