If i say that windsurfing "has a steep learning curve" i mean that to make progress, to move along the X, you must learn faster, move more quickly up the Y. It isnt that you do learn quickly, just that steady progress on one axis requires faster movement on the other. It's really three dimensional: experiance, knowledge, and the speed at which you want to move along those two.
That's not the common usage, though. What I hear most (and usually mean when I use the phrase) is that the initial phases of learning the basics is very difficult, but then after you have those down, moving on through more advanced material is comparatively easy.
