Steep?
String Theory and related math. Math is not my strongest suit, so taking the same courses a decade earlier would have been much easier and more productive.
Some musical education. I have no relative pitch to speak of (or at least a gross mismatch between identification and reproduction), but early training would have at least enabled me to carry a tune without being horribly off key 100% of the time. It sucks that I can't even sing to myself without cringing.
This is the question asked by John Cook on Twitter. He lists responses from different people:
Mine are: quantum mechanics, Python, cooking, the language of philosophy.
What learning curve do you wish you'd climbed sooner? Give reasons and stories if you feel like it. Do you think other people should climb the same curves?