To me, modern quilts are more abstract, less symmetrical, and more "artsy". Traditional quilts are more built on a block repeat, built in rows (either vertical, horizontal, or diagonal). Batiks are an example of a more modern fabric, and fabrics in bright or jewel tones. Just my opinion!
In terms of what people like, well... that will run the scale! I personally tend to favor civil war fabrics (think dark and muddy) and traditional patterns, but there are others who would prefer bright, cheery colors and new techniques. Good luck with whatever you decide.