I understand GavJ's point, and he's technically correct, as far as traditional usage goes. HOWEVER... When the phrase "steep learning curve" is used, it doesn't refer to the same methodology - it's actually reversed. The reason is as follows:
Mountains are steep, and take a lot of skill to climb. Hills, on the other hand, are shallow, and are a relative walk in the park. No pun intended.
Making this graph correct:
It's an interesting point in English. English, like most languages, isn't defined academically, but by what the population as a whole accepts a word or phrase to mean. When the great majority of people say "a steep learning curve", they mean "lots to learn in order to play", making the commonly used definition correct. The important thing is that you are understood, and intuitively, "steep" or "rapid" sounds considerably more difficult than "shallow" or "slow". This is contrary to the traditional academic definition, yes, but most people don't specifically study this, so if you say DF has a shallow learning curve to anyone at all? They'll think it means it's easy.
If you say "steep learning curve" to anyone at all, unless you're talking specifically about academics and not games (and the person you're talking to is aware of it), you're using the definition that defines the phrase as (and it's from the wikipedia article you quoted, GavJ):
Yes, it represents rapid progress, but this is because in order to survive in the game at all, you have to pick things up quickly or you lose... much as in the same way as you need to learn quickly if you're going to go mountain climbing.
Therefore, GavJ is both right, and wrong.
I'm going to duck back out of this now.
But guys, don't fight, and don't pick at other people. That's not how we do things here, right?