Your brain is lazy, shallow, and easily distracted.. but ultimately hackable.
Maximal working memory load is 4-5 things (Cowan 2001) - Even the best students can remember only 6-7 themes out of a possible 30.
We process visual and auditory information separately - bullet-point slides make us read words and listen, overloading of the language areas and leaving the visual cortex absolutely bored, with almost nothing to do. Sparse text with pictures is the best way to present information.
So, don's say much, split the load, and make the brains work - for example, by adding pictures that are not easily linked to the words.
Chris Atherton, a cognitive psychologist (on twitter as @finiteattention, recently delivered this presentation at the Technical Communication UK Conference,