Interesting question. I don’t hang with any history profs here at Big State U. Thoughts…
A good historian (whether a paid degrees professional or not) needs to be a good spy/archeologist, because the foundation of their profession is facts, documents, artifacts, etc. They should be really good at knowing what to look for and where to look, etc. They share some of that with journalism? Archeology?
They need to be good storytellers and pattern matchers, because their ultimate job is to weave a narrative out of some boring appendix of facts. My thinking on academic history writing is that an awful lot of academic writing is awful boring, but it is a necessary intermediate step between a box full of facts and the important history writing, which is stuff you find at a good Barnes and Nobles. After all, what is the difference between writing that is obviously bad, and writing that is correct but unreadable? Nothing, to me.
They especially need the integrity to not draw any inferences beyond what the facts support, or to at least be super clear when they are speculating on a hypothesis. Hypotheses are needed because they steer future searches, but there should always be plural of them.
It reminds me of the difference between statistics and probability. Statistics is about the past, probability is about the future. History is like statistics.