[kurige]

From the Wikipedia page[1]:

The term is often used in common English with the meaning of a difficult initial learning process. Nevertheless, the Oxford English Dictionary, The American Heritage Dictionary of the English Language, and Merriam-Webster’s Collegiate Dictionary define a learning curve as the rate at which skill is acquired, so a steep increase would mean a quick increment of skill.

Arguably, the common English use is due to metaphorical interpretation of the curve as a hill to climb.

[1]: http://en.wikipedia.org/wiki/Learning_curve#.22Steep_learnin...