As an amateur mathematician with an interest in set theory and computability, I have to say JDH has probably advanced my mathematical knowledge in those areas more than anyone else (there are some close seconds!). Perhaps this is due to his prolific use of the site, but more likely I think it is because his writing is clear and he has the ability to provide concise examples that really illustrate a point.
For example, his answer on a question about Goldbach, where some unique insight is added about the arithmetic hierarchy, and a number of examples are provided. Perhaps this is all obvious for anyone that covers this rigorously in a formal education, but I remember this post giving me a sudden understanding of what the arithmetic hierarchy is "used" for (previously only understanding some vague concepts as outlined on the wikipedia page).
And to venture slightly out of bounds, I want to add this clear writing style extends outside of MO, such as some recent lecture notes "A Review of the Godel Fixed Point Theorem" which I will be making use of in a project of my own.
So I'd like to thank JDH for taking the time to answer questions in a clear and accessible manner. Without such kind souls I would be lost still trying to decipher, e.g. Kleene or Davis.
Honorable mention: "The Set theoretic multiverse" was really a joy to read.