What makes a didactically effective lecture? I have made the experience that even the most excellent academics are often didactically mediocre lecturers. I also watched some of last years Nobel lectures and realised many laureates are actually not particularly good at teaching.

What is it that makes some lectures didactically effective and others not, regardless of how knowledgable the lecturer is?
Unless you put a lot of thought into your lectures, it's hard to adopt the mindset of a student who knows much less than you.  You may take certain concepts for granted, and you've long forgotten how you learned them, or what originally confused you.  At the time you learned them you were probably much more well-prepared than the average student, so common stumbling blocks for students were never issues for you.

This means there are a lot of professors who are good at giving talks (to people well-versed in their field), but poor at giving lectures (to people with little or no background).