Games are sometimes held back by the lowest common denominator, the weakest system they release on.
So, if 343 are trying to get Halo Infinite to run at 1080p 60fps on base Xbox One with an open world design, then obviously graphics quality would need to be lower than an open world game that was aiming for say 900p 30 fps on base Xbox One. I do think that is the crux of the problem here most likely, 343 decided that they wanted 1080p 60 fps on base Xbox One, instead of aiming for something less demanding like 1080p 30 fps, 900p 60 fps, 900p 30 fps, or 720p 60 fps. As a result of that decision, graphics had to be toned back enough to hit that resolution and framerate on base Xbox One, which in turn will have also lowered the graphics quality all the way up the rest of the hardware lineup, from Xbox One X and mid range PC's, to Xbox Series X and high end PC's.
I really don't think the base Xbox One Model support is the problem here, because I fully expect the Xbox one version to be 720P 60FPS with smaller draw distance.
I think the prime reason for the odd graphics are caused by the use of dynamic lightning like digital foundry pointed out. I'll will add to this that dynamic lighting prevent the use of sub subsurface scattering which would greatly help model feel less plastic. Classic halo artstyle might also exacerbate visual oddities.
There's also the fact that we have seen only one Biome which felt empty, previous trailer have shown other environments, some with more vegetations.
Pop-in was also a problem but I fully expect this to be fixed by released.
The question here is would I want better visual over dynamic lightning? I need to see more of the game before I made my decision on this.