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Ultimately, all real teaching beyond that of early childhood (when the child is passively being 'filled' - or inculcated - with that which society deems necessary) depends on motivation : the pupil must want to learn.
This applies very obviously to lectures. For lectures to work, to be effective, depends on the class being motivated to learn.
If the class is not motivated then they will not learn - they will daydream, doodle, flirt, engage in social networking, surf the internet... anything other than the cognitively-difficult business of paying attention and striving to understand.
If the class is not motivated, then the only way that attention can be compelled is either by terror (notably fear of failing exams and being thrown out/ disgraced) or by entertainment.
So the degenerate phenomenon of TED talks is what happens when the lecture degenerates to the point of mere entertainment - amusing, shocking, or exciting five minute sound-bites - desperately striving to grab and hold an unmotivated audience's attention. In a word: degenerate. (Or have I already said that?)
At a lower level, the need to entertain is what drives lecturers to use visual aids and multi-media - so the class sits and watches video or movie segments instead of learning.
Or, perhaps there are innovative 'teaching' methods such as getting the class to chat among themselves (break-out sessions), have votes, or do pretty much anything except learn the material - for which they have zero interest.
In sum - there are situations, many situations, in fact most situations in modern middle and higher education - in which effective lectures cannot be given, because the class does not want to know.
In those situations, lectures are perhaps no worse than the alternatives - but the whole thing is a dishonest waste of time, energy and resources; no teaching can occur, because nobody really wants it to happen (or, at least, the class don't want it enough to overcome their own natural tendencies to idleness and distractability).
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Note added: My primary objection to TED talks is actually not that they have nothing to do with education; but that they are media for the mass propagation of vomit-inducing smugness.
