A lecture should be interesting; otherwise, the lecturer is wasting everyone's time. If it's not interesting, people will not learn. Given that the obvious purpose of a lecture is to teach and educate (note that many lectures do not have this purpose), people need to be interested in the material to pay full attention.

What this means is that the lecturer must not just know about the subject; they must also be excited about it, and jumping at the chance to teach it. This applies in general -- if you're going to talk about something, or even node about it, make sure you have some desire to do so besides just "giving a talk" or "gaining writeups". Your enthusiasm will be reflected in the attention of those you are communicating with.