The photography
While this film was made on Technicolor film, there were times when the images appeared to me to be on black/white film. So many "dark" scenes of Llewyn's despair appear B/W. Subdued lighting characterizes so many scenes. It's like "Bonnie and Clyde" for me, which I'm "sure" was in B/W to reflect the era, and then have to force myself to realize it is actually in color.
I have never heard of the cinematographer on this film before. But, the photography was superb. Noted British cinematographer Roger Deakins has done Coen films, but was unavailable for this one. The Coens obviously were not going to pick just anyone to film this. Their alternate choice here was perfect. I'm going to look at this DP's other work to see what it's like.
E pluribus unum