I mostly buy patterns for quilts that I can't figure out myself. For example, I just finished the Gravity quilt by Jaybird Quilts. The pattern for it was a book (too much to work out on my own), but very clearly laid out. There were lots of diagrams, and just enough text to explain. The text was simply written and not crowded. I would consider this an example of an extremely good pattern.
I don't like it when I have to "decode" a pattern before I can start on it. I like Judy Neimeyer quilts, but I hate her patterns - the text is crammed together, and everything is number/letter coded in a way that I don't find intuitive.