You don't want to have the user click - and you don't know how long the user will want to look at the slide, so don't want to use a timer?

Ummmmm - how then are you going to choose when the next slide should be shown???

Unless you have an event to trigger the next slide - frankly it's impossible (for a few more years anyway ... until they add mind reading to Office - LOL!)

Basically you have three choices to move to the next slide:

1. user clicks
2. timer action takes place
3. action button on slide is clicked (only one real difference from option one - this allows the user to understand that there is no timer and they must trigger the event!)

On the Slide Show tab there is an option about rehearse timings - but again, this uses a timing approach.
Even all the VBA Options will lead to a timing of the move!

Sorry, there is just no way round it