Being able to explain a concept in the most simple case clearly and concisely (not an analogy either). Before complications, before generalities, before even notation, explain the concept and understanding the concept.
Like take the integral and derivative as a very basic case. Understanding quite simply what they are and a rough method for calculating them and then explaining those methods is the mark of intelligence. Rigor, proof, and generalization comes afterwards.
Some concepts do require a lot of foundational knowledge, but you should still be able to explain them clearly assuming your reader or listener has that foundational knowledge. I often get sick and tired of reading texts and reports and papers that go far too deep into the trivial overcomplications of what they're studying. You only overcomplicate when it is absolutely necessary. >>13889566
Number two really is the key for me. There are underlying similarities in problems due to math. Once you unlock that pattern sight, problems become MUCH simpler.