Answer:
Actually, it has little to do with the learner and more to do with how much there is to be learned. The steeper the learning curve, the progressively more difficult the concepts to be mastered are.
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Additional: The metaphor "steep learning curve" originally came as a positive reference. A steep learning curve meant that one became quite proficient with a minimum amount of effort/time. It LATER came to be used in the opposite sense - as a negative reference to something difficult to learn (which actually is a shallow, not steep, curve!).
A learning curve is shown as a graph of "amount of learning" in the Y axis and the "amount of time or effort" in the X direction. A relatively 'normal' learning curve would be a sloping 's' curve, with its tail starting at the lower left and progressing to the upper right where the head of the s lies.
In common (technically incorrect) usage, "steep learning curve" is meant to indicate that to learn the subject/technique takes a long time and is difficult.
For a cogent explanation (but unfortunately without pics) see
http://en.wikipedia.org/wiki/Learning_curve#Common_terms
Best 'pic' I can make with this limited editor (ignore the ·∙∙ ):
Proficiency
|∙ ·∙∙ ·∙∙ ___100%
|·∙∙ ·∙∙ X
|∙ ·∙∙ X·∙∙ ·∙∙ ·∙∙ ·∙∙ (a quick/rapid proficiency, a steep slope, = easy)
|·∙∙ X
|∙ X
|X____________
Time
Proficiency
|∙∙· ∙·∙ ·∙∙ ∙∙· ∙·∙ ·∙∙ ∙∙· ∙∙· ·∙∙ ·∙∙ ·∙∙ ___100%·∙∙ ·∙∙ ·∙∙(more time reach 100%)
| ∙·∙ ·∙∙ ∙∙· ∙·∙ ·∙∙ ·∙∙ ·∙∙ ·∙∙ ·∙∙ ∙∙·X
| ∙·∙ ·∙∙ ∙∙· ∙·∙ ·∙∙ ·∙∙ ∙∙·X ·∙∙ ·∙∙ ·∙∙ (a slow proficiency, a gentle slope, = difficult)
| ∙∙· ∙∙· ∙∙· X
|X
|_________________________________
Time