If I am shopping for fun and don't really need anything, I usually shop for price--what's on clearance, is there a sale, etc. I also look to find good lights and darks, as another poster mentioned, because like many people I tend to have too many middle value fabrics. In general, I am always looking to expand my use of color. I am trying to "lighten up" my color choices. I use solids frequently because they are economical, but I do buy good brands.
I was recently interested in trying some Amy Butler fabrics. I was able to find a good deal at fabric.com using a sale (25% off) and free shipping and got the fabrics for about 33% off retail. I think that's a good deal. And I didn't use any gas! The fabrics are good quality too.
I also love orange fabrics and the colors that go well with them. Also turquoise and greens.