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...
It has nothing to do with driving..
* http://www.salon.com/2013/08/22/according_to_the_dictionary_...
That's the 'informal use', as in, everyday communications such as, but not limited to comments in online communities.
The "informal" usage stems from people not knowing what they are talking about. No need to perpetuate it if you know better.