To clarify, I mean stuff that is maybe not immediately obvious to someone who wants to learn about puzzle design. For example, A common thing I see is that a lot of great puzzles are not actually one simple problem, but can be broken down into several smaller mini-puzzles.
And by perfect, I mean it cannot reasonably be improved upon. A puzzle where nuance and tile is accounted for (at least for the intended solution) is a perfect puzzle in my book.