I've seen some criticism of the new poster on grounds like the Vader helmet being off-center and the lighting on the faces being weird and the like. But it seems to me that the new poster cleverly emulates a few design quirks of the very first Star Wars poster ever, the one painted back in 1977 by the Brothers Hildebrandt. I place them side-by-side below:
The off-set Vader helmet provides a bookend of sorts to the offset Vader helmet, facing the opposite direction, in the original poster, and the "starburst" surrounding the clashing lightsabers in the Revenge of the Sith poster echoes the "starburst" in Luke's lightsaber in the Hildebrandt poster. Also, the light pattern in those clashing lightsabers forms a circle, with a second, offset circle in the upper half, suggesting the Death Star.
So, it seems to me, the poster for the last ever Star Wars movie at least partially hints at that classic first-ever poster for the first-ever Star Wars movie. Very cool.
(I squashed the Episode III poster a little to get the images to line up correctly.)