Written in red in the Altar Book and hymnal (many more in the Altar Book than the hymnal). They are everyone's favorite whipping boy. But they get a bum rap. I love how the intro to the Lutheran Service Book: Altar Book puts it:
Rubrics are never to be seen as an end in and of themselves. Their purpose is not to call attention either to themselves or to those leading the service. The purpose of the rubrics is to promote reverence toward God, thus freeing the worshiper to receive God's gifts offered through His means of grace, and to offer Him the sacrifice of thanksgiving and praise. A genuine and authentic use of the rubrics allows the focus of the service to be on the grace and mercy of the triune God, Father, Son, and Holy Spirit. (p. x.)
In other words, they delineate good habit. So you don't have to think about them. So you don't have to think about what to do at this point or that, which way to turn, what gesture to make with your hands. They become second nature and instead of looking like a "performance" they become simply how we live together in the Divine Service.
When rubrics are all made into "may" rubrics - when everything is treated as optional - the result is that no one knows what to expect or how to act. The pastor moves onto "center stage" and he holds the service together by his antics. Rubrics are there so that you don't have to think about such things - the focus is solidly on the Lord, His Word, His gifts. For the parish that respects and observes them, the pastor's actions become totally unremarkable.
Rubrics. They're all about getting the spotlight off the presider and letting it stay fixed on our Lord who comes among us with His gracious words of life and with His body and blood for our forgiveness, our life, and our salvation.