I repeateadly hear appalling comments along the lines of "everybody should be able to follow the 15 first minutes of your presentation", or even worse "it is OK if only 5% of the audience follow the last 15 minutes of your presentation". Let me be clear: it is not.
The main goal of a presentation should be to ensure that the members of the audience have a good time (probably by learning something new). There is nothing worse than being lost in a never-ending talk. Therefore, my number one goal when preparing a presentation is:
Everybody should be able to follow everything.
In particular, a definition, figure, or proof should appear only if I will be able to guide the audience through it in detail.