While their lack of concern about keeping visuals consistent from the original trilogy (see the various visual retcons in Halo 4) is my main beef with 343I's art department, my other issue is that they lack Bungie's approach to aesthetics in general. The color palette used in 343-era games is much more limited than what we saw in Bungie-era Halo games. Everything looks so dull and muddy, with muted colors that tend more towards the genre standard of "Real is Brown," or are darker in general, or just look fuzzy and oversaturated (not sure if I'm using the right terminology for the last one). Even Bungie-era maps with lots of brown or grey managed to look cleaner and at least a little bit more colorful. I mean just look at much more colorful Coagulation is than Bloodline:

And how Sanctuary has touches of green trees and a blue sky, contrasted against the near-uniform brownness of Shrine:

And how blue Lockout is compared the unnatural glare of peach-ish lighting seen on Lockdown:

And how the purples and other hues of Midship, Heretic, and Zealot contrast to the Halo 5 remake of Midship, which has the purples swamped out by dark, gloomy blue light that dominates everything:

And, well, you get the point. 343I-era Halo more often than not just isn't pleasant to look at. Cleaner lines, more vibrant colors or at least colors that contrast better, less harsh lighting. These things would help make 343I's visual assets easier on the eyes.