While I rarely use a pattern, when I do I look for things you all have already commented on. Straight forward clear directions, good illustrations and because I am a bit anal about trimming I hope for a statement "square unit up to". Yardage for me is only a guideline - I have had so many "patterns" that overstated yardage. I do what Barb in Louisiana does - a lot of times - design it in EQ and go from there. I have certain way I do things such as HSTs and Flying Geese and I adapt the construction to accommodate my skills and ways.
That all being said ... if I were designing a quilt pattern for market, I might include a sheet of "basics" such as 1/4" seams, pressing tips, HSTs, QSTs etc.... Then in the pattern itself I would assume they understood the basics. I have noticed that a lot of quilting books do this and it is great as if you are curious as to how they assemble or do something there is a reference sheet if not you can plod ahead.