Volumetric lighting for starters, the god rays look like they are rendering in 480p on 1X even if the overall image output is native 4K. Water reflections are pretty terrible too.
Draw Distance is the biggy.
Stacked up against the PC release, it's running a combination of lower than low, low, and medium settings. Textures are at Ultra. I feel like going for a full fat 3840x2160 was kind of a waste when other areas could have been improved on.
RDR 2's IQ and TAA was made to be played at 4K though, I've played it on PC and it looks pretty horrible at times in motion at 1080p/1440p. Overall they did a good job with the 1X version outside of the lighting quality and water. Before the PC version existed everyone was saying the draw distance was fantastic on 1X.