Colors, shapes, sizes.
In a discussion of possible fabrics for Bonnie Hunter's new mystery, someone mentioned that they prefer more modern looking quilts, then showed 2 possible fabric pulls. One was in the suggested colors (indigo, red, light blue + neutrals). The other was yellows, greys, black and light neutrals. The second grouping definitely had a more modern feel, even though the reds and blues of the first grouping were more modern prints (as opposed to calicos, for example).
I find patterns for modern quilts have, in general, larger (and therefore fewer) pieces. Chunkier squares and rectangles with some triangles, rarely stars - a mainstay of many more traditional quilts.