One thing that has struck me while reading through The Noble Eightfold Path and In The Buddha's Words is that, although there is an outer simplicity to the Buddhist Eightfold Path, things get really complicated really quickly. The Dhamma (the entire Buddhist teaching) is much more than just the Eightfold Path.
Here are some quotations from Bhikkhu Bodhi's introduction to the chapter Shining the Light of Wisdom in In The Buddha's Words.
"The Five Aggregates. The five aggregates are the main categories the Nikayas use to analyze human experience. The five are: (1) form, the physical component of experience; (2) feeling the 'affective tone' of experience – either pleasant, painful, or neutral; (3) perception, the identification of things through their distinctive marks and features; (4) volitional formations, a term for the multifarious mental factors involving volition, choice, and intention; and (5) consciousness, cognition arisen through any of the six sense faculties – eye, ear, nose, tongue, body, and mind. Examination of the five aggregates...is critical to the Buddha's teaching..."
"The Six Sense Bases. The Salayatanasamyutta, the Connected Discourses on the Six Sense Bases, contains over two hundred short suttas on the sense bases. The six internal and external sense bases provide a perspective on the totality of experience different from, but complementary to, the perspective provided by the aggregates."
"The Elements. The elements are the subject of the Dhatusamyutta. The word 'elements' is applied to several quite disparate groups of phenomena, and thus the suttas in this chapter fall into separate clusters with little in common but their concern with entities called elements. The most important groups consist of eighteen, four, and six elements. The eighteen elements are an elaboration of the twelve sense bases..."
"Dependent Origination. Dependent origination is so central to the Buddha's teaching that the Buddha said: 'One who sees dependent origination sees the Dhamma, and one who sees the Dhamma sees dependent origination.' The ultimate purpose of the teaching on dependent origination is to reveal the conditions that sustain the round of rebirths and thereby to show what must be done to gain release from the round. To win deliverance is a matter of unraveling the causal pattern that underlies our bondage, and this process begins with understanding the causal pattern itself. It is dependent origination that defines this causal pattern. An entire chapter of the Samyutta Nikaya, the Nidanasamyutta, is devoted to dependent origination. The doctrine is usually expounded as a sequence of twelve factors joined into a chain of eleven propositions..."
Five aggregates, six sense bases, eighteen elements, twelve factors, eleven propositions. One could go on. This kind of "hyperclassification" seems to be prevalent in other forms of Indian philosophy as well, so I don't think it's necessarily unique to Buddhism. But it does get complicated.
It also stands out to me that the rhetoric of the Pali Canon lends itself to statements like this: "One who sees dependent origination sees the Dhamma, and one who sees the Dhamma sees dependent origination." You continue to find statements that imply, "If you just get this, you'll get the entire teaching." Or, "The Dhamma is summed up in this." Unfortunately that thought is applied to many concepts within the Canon. I've come to just see this as part of the rhetoric to be aware of. But it continues to make me confused as to what is, and what is not, essential in the teaching.