What do you consider sound Christian orthodoxy?
That depends on what perspective you are coming from. In the Reformational tradition, the churches are usually confessional, which means they subscribe to a historic confession of faith that they find to be a sound summary of Biblical teaching. Baptists usually subscribe to the 1644 or 1689 London Confession; Presbyterians subscribe to the Westminster Standards; Continental Reformed believers subscribe to the Three Forms of Unity (Heidelberg Catechism, Belgic Confession, Canons of Dordt); and other may subscribe to the Helvetic Confession. Those confessions contain articles on all of the major aspects of the Christian faith that would need to be defended. Obviously, however, these confessions aren't the bounds of salvation, since they even differ among themselves (especially the Baptist). If one was confessional, I would suspect they would be firmly grounded in all of those foundational topics, and be ready to give a defense for those beliefs.
A more basic outline of Christian belief is found in the "Apostle's", Nicene, and Athanasian creeds. However, false teachings have required more thorough outlines of Christian belief to be outline.
I believe the purposes of the historic confessions were/are to give a written document that will eliminate ambiguity of belief amongst the members and outsiders. Also, it provides a helpful educational resource for both adult and child.
I personally find the 1644 London Baptist Confession to be a sound summary of Biblical teaching, however, the confession itself has no article on the Lord's Supper - though Benjamin Cox's appendix to the confession does contain an article on the topic.
If you are not subscribed to a confession, I don't know what your bounds are. One could respond with a simple "The Bible is my confession". However, the Mormons and the Watchtower organization (and most other cults and heresies) make the same claim. The question is "how do you interpret the Bible?" That isn't to say that one that is not confessional can not have boundries - they certainly can - but they would have to be arrived at on a more individual basis, and I personally always try to avoid individualism if possible.
Honestly, everything the Lord taught us is "essential". Yet, there are doctrines that need to be insisted upon with more ferver than others, such as the person of Christ, the atonement, the Trinity, and the nature of the church.
Hope this is somewhat helpful.
With care in Christ,
Taylor