Ask HN: What are your issues with public speaking?

I'm about to start a public speaking coaching business that would be mostly dedicated to introverts in the tech / science fields, and I would like some suggestions.

As a guy in these fields myself, I have seen many conference presentations and speeches that failed, for several reasons. I know several business opportunities that have been lost after crappy presentations.

So, fellow hackers: what are your biggest issues, regarding public speaking, if any? What do you struggle with? What do you dislike? If you could magically improve anything, whether it's the actual content of your speech, your slides, the way you speak, your posture, I don't know... What would it be? And what do you usually like or hate in the presentations you see?