One would be the adroit instantiation of classes within the design-space that ensures each major niche is occupied, informing in a very deliberate fashion their features at various levels. An interesting fingerprint of which is seen in the generally uniform power watersheds and its highlight by rogues as an exception. The way diverse approaches to combat are instanced in class basics, features and feats is also a nice piece of work and subtle enough that one rarely sees all its pillars discussed. Sneaking Bo9S in as a subclass was rather cute!
Two others I find fascinating are the writing of fiction-first ideas into the basic pattern and the system for ability checks, and the faint nods to story-now in TIBFs and Inspiration. Speaking of backgrounds, the game purpose of backgrounds (in distributing skills) is rather neatly disguised as colour. It's intriguing to observe a nod to FitM in bardic inspiration, or the neat little piece of ludic self-awareness in the divination mechanic.
The technical language (the legal wording of rules) is remarkably consistent all through, and continually echoes specific themes. (DM power being one of them.) 5e is a sophisticated and amazingly robust piece of design given it's breadth and ambition. One may well feel vexed at its centricity - or refusal to take a stand - while still admiring the feat of designer engineering that accomplishes it.
Anyway, I am sure that the perceptive reader will be able to find the many more subtle arrangements, threads, and patterns throughout the rules
