To nitpick: almost everyone I've heard use this in casual conversation (who isn't nitpicking) uses "steep learning curve" as in physical analogy, e.g. bicycling

To nitpick: almost everyone I've heard use this in casual conversation (who isn't nitpicking) uses "steep learning curve" as in physical analogy, e.g. bicycling to a goal, rather than describing a function plot. A "steep" path is harder going than a "flat" or "shallow" one.