Statistics. I took it in High school, but it was so poorly taught that I learned almost nothing from it. Now I use statistics every day.
Scripting Languages. I learned Java in High School and rode that knowledge all the way through my CS degree. But when I got a job as a software engineer, Java was among the worst languages available for solving the type of problems with which I was dealing. Looking back at my old code, I could have saved hundreds of hours if I had learned Python instead of Java.
Related, curves I haven't climbed but wish I could/would/intended to:
A European foreign language. I dabbled in a variety of languages in school and settled on Chinese, wasting many years and having little to show for it. If it had been an Alphabetic language (and better yet, a Latin-alphabet language), I'd have a much higher level of proficiency.
A Martial Art. I love the martial arts, but I've never been able to devote myself to any one of them.
Seconding statistics.
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?