To me, it's the quilted motifs that make them instantly recognizable as "Welsh" quilts, as opposed to American whole cloth or Amish quilts.
More "earthy" looking.
Love them.
Here's a fabulous blog devoted to the style:
http://welshquilts.blogspot.com/
Here's a great book on the subject:
http://www.amazon.com/Making-Welsh-Quilts-Mary-Jenkins/dp/0896892549/
And here's the QB's resident hand quilting expert (at least I think so), BorntoHandQuilt, with a better explantion in a post from last year (btw, the whole thread was full of great information on wholecloths, etc):
http://http://www.quiltingboard.com/...ml#post3471409