Many people refer to things that are difficult to learn as having a "steep learning curve.

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Note added at 1 hr (2006-04-18 09:54:11 GMT)
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It is a cliché today to refer to a “steep learning curve” to indicate that something is difficult to learn. In practice, a curve of the amount learned against the number of trials (in experiments) or over time (in reality) is just the opposite: if something is difficult, the line rises slowly or shallowly. So the steep curve refers to the demands of the task rather than a description of the process.

http://www.dmu.ac.uk/~jamesa/learning/lerncrv.htm

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Note added at 8 days (2006-04-27 02:40:38 GMT)
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这个短语被大家惯用做difficult to learn的意义，是由于很多人都有望文生义的习惯，以致以讹传讹，使这个短语失去了其真意（即我引用的网站中的专家对此短语的正解），中文中这样被大家广泛误用的例子也不少，如“首当其冲”之类。在这样的情况中，我认为翻译时应当根据context来分析作者本意，虽然大家的惯用意义与原意有所差别，可翻译者也之能将错就错了。这是一篇一般性文章，作者不太可能是教育学家，没有上下文，我也只好假设这个作者是使用这个cliche并难逃讹传之网的大多数人了。