The Librarian
Banned
You could probably fit a third to half of "The Silent Cartographer" into the Battle of The Citadel.
Night doesn't look like night, too. Two Betrayals felt like you were still fighting in the daylight.I never finished CEA. So that noted, I did think a lot of the art was a turn for the worse. Two examples are the way dark areas (such as the light bridge area of the second mission) were brightened up to the point where you could see the whole space clearly, no flashlight needed. Totally robbed the scene of the atmosphere it held. And a lot of detailed ornamental texture work (most notably on Silent Cartographer) was replaced with simple repeating hexagons.
Later things like the removal of fog on 343 Guilty Spark hurt the rich atmosphere Bungie created as well.
Now as you said this stuff is very subjective. A lot of people liked those changes. But they are ones that for me at least were a big step back. I played many sections with the original graphics instead, which to be fair is a great feature.