Looking back at recent client work, the V1.0 briefing decks I saw, here are some of the mistakes I encountered. Not a complete list, not in the right order, just some examples that came across my desk the past few weeks.

• Without even reading a single slide, some presentations have an incredibly dated look & feel: standard PowerPoint colors, old, low res, cliche images, even clip art. Look at a presentation you think look good, and simply copy the style elements with the colors of your logo and your presentation will dramatically improve, without changing one bit of its content. And by copying, I mean copying. Look at the positioning of titles and boxes, the amount of white space that is used. With an example design in front of you, your page should be able to look exactly the same, no excuses!
• You have gotten used to the strategy review slides that have been in all the decks since you started out a couple of months ago. The audience sees them for the first time. Charts you used to evaluate your strategy, and not charts that can be used to pitch investors or customers. These presentations look highly professional, are loaded with content, say all the rights things, but do not change heart and minds.
• Some presentations look too cute, minimalist infographics, icons, and cartoon characters. It is maybe nice and funny for a 1 minute product video, but people need to feel that there is a proper business behind this. If it fits your brand, it is fine to use the "cute" story as a sales presentation inside your investor deck, but the other slides should provide a bit more gravitas.
• There are presentations which take too long to get to the point. Endless series of charts with market backgrounds, often providing statistics for trends everybody pretty much agrees with. Cut to the chase earlier, and if required, put a quick placeholder slide to remind the audience of the market context you are working in
• Most of the time, I am not to strict with structure in a presentation (yes, I am an engineer who used to work for McKinsey). In 20 minutes, a story that flows nicely is more interesting that a highly structured presentation full of tracker pages and sections. Having said that, some companies actually need it. If you are a biotech with a significant portfolio of drugs in development, the audience need to stay on top of what we are actually talking about right now.
• Dwelling forever about your technology, or doing a very long product demo does not help, but the other extreme is equally worse: summarizing your product in 2 sentences. The audience need to "feel" what the current pain is, how brilliant your product is, and how clever the underlying technology is put together. Each individual case has a different balance of these 3 points.
• Certain businesses are basically apps. When the app is still in development, crafting a grand pitch deck might be overkill. Money could be better spent on some great-looking screen mock ups to explain investors or potential partners what it is you are actually doing. Graphics quality is not super important here, you can include 1-2 screens that show that your company is able to produce a good looking product. What is more important is the flow of the application demo you are showing. In these cases, designing the app experience, and designing the pitch deck can go hand in hand.

Some examples from recent incoming work, maybe it contains some useful advice for your own presentation.