Part of what makes Photoshop such an incredibly powerful and pleasurable program to use is its well-thought out implementation: the interface is polished and uncluttered and easily customizable (despite its vast arsenal of tools and commands).
In fact, there is a surprising amount of "hidden" functionality within Photoshop; so much so, that despite the number of years you work with the program, you will undoubtedly keep discovering more and more about it.
I doubt anyone could claim that they know "everything" about Photoshop (except maybe Adobe's software engineers).