That's what I understand by a "learning curve" too. I would tend to agree with the wikipedia proposed explanation: "steep" is, in some contexts, synonymous with "difficult". Saying "you have to climb a shallow learning curve" would certainly be interpreted wrongly by most people.
Concerning the OP question, I wish I had learnt something interesting at school, instead of the thoroughly irrelevant, utterly boring, mindboggingly wrong pseudoknowledge.
To zedzed and D_Malik: I think what you have in mind is learning to play an instrument as a profession. I that case I agree that the rewards are probably not commensurate to the effort, unless you are exceedingly talented and lucky. But for everybody else it's probably a good idea. If done for its entertainment value it's surely worth it, and it helps you understand and enjoy music at a deeper level. On the minus side, it also makes you realize that most popular music sucks. Really really sucks, to the point of irritating me.
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?