Many math instructors are trying to teach today's students using only yesterdays tools and approaches. And neither the instructors nor the students pleased with the results. But the problem with introductory lectures is that they are full of words that have not yet taken on meaning and full of answers to questions not yet asked by the students. A further problem is that many lecturers fall into the trap of believing that their job is to think for the students. This effectively shunts the students to the sidelines - making them into mere scribes who verify in the homework and tests the math truths promulgated by the lecturer.
Books compensate by giving samples of how to solve every type of homework problem.
Professors compensate by giving homework and tests that are much easier than the material 'covered' in the course, and then grading the homework and tests on a scale that requires little understanding. Outsiders are amazed at this phenomenon, but within the mathematical community, we dismiss it with shrugs. In summary, I do not disagree with SK's approach to lectures, as he gives some great advice, which I used to follow as well. However, I do question the necessity, importance, and educational quality of lectures as a method for students to learn mathematics. Do you explain that a cat is a relatively small, primarily carnivorous mammal with rectile claws, a distinctive sonic output, etc.