You cannot appreciate Shakespeare's Henry IV, unless you also consider the historical context in which it was written, and you will not laugh at a joke, unless you understand the culture in which the joke was written. There are specific and definable prerequisites for every skill or knowledge. Excellent educational software will evaluate whether the learner has the prerequisite knowledge for a concept and help to fill the gap. Thorough Treatment of the Content Of all the ills of bad education, one of the most ubiquitous is that the content is often incomplete. Very few authors either have the time, the inclination, or the techniques, to prevent this.
Incomplete content is the main reason why learning is often so difficult.
For example, a teacher might describe the formula for acceleration, forgetting to define acceleration. Such an omission can be disastrous because the student ends up memorizing the formula without ever understanding what it really means. The student has learned by rote, and will never be able to apply this information in a creative, problem solving, situation. Thorough treatment of the content means that such information holes are resolved. Students cannot be expected to be clairvoyant, and to fill the gaps themselves.