Short answer: The presenter does not think about what the audience needs to hear to understand him/her. Unfortunately this is an extremely difficult skill to master - requiring one to step into the shoes of someone in the audience. But that's not even the main problem.....
The main problem is that most presenters are not even aware that this is an issue. Thus they never get better, even after hundreds of presentations, because they just don't know this simple fact. They think that presenting their latest-greatest data would surely dazzle the audience, right? Wrong. The most common problem in a talk is the presentation of the cart well before the horse, with a perfectly organized horse and cart picture in only one person's mind - the presenter's.
Here's a typical scenario: Presenter puts cart before horse the first time --> "Joe the Scientist" in the audience struggles fiercely for several minutes to understand ---> By the time Joe is on board, presenter is 5 slides ahead ---> Joe REALLY tries again, and FINALLY gets it ---> now presenter is 15 slides ahead. Though Joe came to the talk in complete earnest, he gives up. Starts thinking about own projects or sneaks away.
Here's what I do. Prepare the talk 2-3 days in advance and pretend I am "Joe the Scientist" looking at the talk. Put forth a slide, mutter what you would say about that slide to Joe, and see if Joe understands. Does Joe have reasonable questions that you have not answered? Are you assuming Joe knows things that he really does not? Then the slide needs more work. DO NOT go on to the next slide until Joe is happy. Keep doing this until Joe is a satisfied customer! For me, usually Joe is never absolutely happy, but its time to give the talk anyway...
Some old ones Reposted...
8/18/13 - A Classification of PI's - find yours' in the list!
7/6/12 - Way to a "Science paper"
5/30/11 - 20/20 Hindsights
9/30/11 - Evolution of a bizzare, new Idea
11/17/2011 - Rationale for curiosty-driven research...
lessons from a 4 year old
1/28/12 - "GTFM" - hilarious article on grant writing!
The PI Blog
This blog exists because my wife seemed a bit tired of being the only recipient of my random pontifications on life and Science for many years; and gently encouraged me to vent in a blog instead. From time to time, I put down thoughts that occur to me as I naiively stumble through a life in Science - bestowed upon me by accident (literally!). Please keep in mind that these musings are rather obvious things of little or no use to anyone, and are certainly not personally targeted in any way, even though they are obviously derived from my experiences. OK, enough said.