The trend to over-share might be in part because for authors there can be quite a bit of fear-driven pressure [from peers and self] to be doing what the most visible authors are doing. And as many commenters have pointed out, some very visible authors fall into the category of internet over-share in terms of anything goes personal information. Some put out a lot of information very naturally, and some really very entertainingly - but others I suspect may be acting somewhat out of character in order to try to meet a perceived expectation [besides maintaining blog posting volume]. These can be our most prominent examples.Actually, all of the examples I've witnessed were in person, at publisher-organized events. AND several of the worst offenders were very established writers who could easily have refused to do events. I think some people just have no damn filter.
In terms of the dangers of speaking over-share, when people ask authors to speak they often don't have any kind of request for a particular topic or theme, and we are just expected to do 'our talk'. I've seen authors talk about all sorts of things, more and less successfully. So besides what not to say [which IS helpful!!], what would you give as advice for an author wondering what exactly to really focus on when asked to give a general talk?Well, the reason authors usually aren't given a topic or theme is because nobody else but you knows what you can speak entertainingly about. You could speak about eggplants and how they were clearly never meant for consumption. You could speak about your first meaningful experience of the power of storytelling (as long as you aren't breaking any of the rules). You could speak about publishing's pitfalls or writing's ecstasies or vice versa. You could even speak about your book, if you really want to.
The trick is just to find something that you want to talk about and other people want to hear. Still, if all you can manage is to avoid speaking about something your audience finds uncomfortable, disgusting, inappropriate, weepingly dull, or nightmare-inducing, I say: that's a job well done.