It's all about contrast! Value (light to dark) is one kind of contrast.
I also like to have scale contrast - small scale prints, some medium, and some larger (and maybe a more neutral background). A quilt made with all small scale prints looks prim and dull. A quilt with all large scale is "mushy" - they all blend together. But if you have a good combination of different sized prints, the quilt tends to be more interesting and it "keeps the eye moving".