Something that stood out from a bad presentation I had to sit through recently was the speaker's insistence on explaining to us what was coming next, why she chose a photo, where it was taken, that she'd tested the presentation in front of friends before (which is recommendable, of course), etc. This sort of meta information seemed very important to her but didn't really help the audience in understanding the presentation any better. In other words, it was extraneous information that only distracted from the actual presentation.
There's an old list of tips for writing for the web, by non other than Tim Berners-Lee himself. One of those tips is "Don't mention the mechanics." By which he means that you shouldn't write something like "You can find more information by clicking on the following link: (link)". That's mentioning the mechanics; the clicking and the link. Instead, you should write a normal, natural sentence and make part of that sentence the link. Something like "John Doe has (more information on the topic)".
In a way, this principle also applies to presentations: Don't mention the mechanics of the presentation. Tell us a story and let things come up naturally. Use an agenda slide only if you must (for an unusually long presentation, for example) but otherwise, structure your presentation such that the structure becomes obvious just by listening to it.
Adding meta information will throw the audience out of the flow of your presentation. It's like stopping a movie to explain that the following scene is very important for the rest of the story. If that's the case, the director should make it obvious for the viewer with some of the tried and tested techniques (dramatic music, change of colour or lighting, etc.); but don't throw us out of the story.
(Image Credit: Cogs by Arthur John Picton, from Flickr)
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