"Do not go beyond what is written" is a most profound statement. And it argues for:
- Less clear doctrine than more clear doctrine.
- Less emphasis on what can be derived from rather than what is simply said.
- Less speculation as to how spiritual practice "X" should look and more following of Christ.
And we tend to get so worried about how to follow Christ in deciding between non-sinful choice A over non-sinful choice B rather than in living in the manner that we were created to live (and for that matter, commanded to live — by both the law and by Christ).
How we should live is written. But what non-sinful choice we should make is not. But we worry about whether we are living right by the right source rather than worrying that we are not living right. The former is really not written. The latter is.