For me it's a few things –
The lighting looks superb (and it had better be, so big!)... especially the cathedral shot, holy cow.
The sheer number of NPCs, which I think has definitely informed the look of the whole thing.
And finally.. I will catch some hell for this... but I do believe they are trying for a softer, more painterly look to their image. That's what I get from it (it's very period-appropriate), and I think it's a good choice artistically, especially when you know you won't have native rez on your side. Better to lean into it. That sounds like the famous 'subtle beauty' Halo comment but, well, there it is.
If I had a little more time I'd dig up some paintings from that period and compare the style.
AC4 had a kind of super-high-contrast sharpness to the palette that was always a bit off putting to me. That phenomenon where the main character can be standing in full blasting daylight and still have parts of his costume or appearance descend all the way down to pure black.