That question is a big compliment for your PowerPoint presentation: you have succeeded in making your PowerPoint not look like PowerPoint. Here are some simple steps that can help you:
- No hierarchical bullet point texts (if you have to put three messages use 3 grey boxes with a short sentence)
- Switch the standard Microsoft Office font Calibri typeface for Arial (other exotic fonts will cause issues on tablets)
- Avoid the standard Office colours (blue, faded red, faded olive) and use your own colour palette, also in data charts
- No dirty gradients, drop shadows
- No heavy graphics and/or colours behind the title or at the bottom of your slides
- Create many slides with page-filling images
- Remove the default clutter of data charts (tick marks, etc.)
The same applies to Apple Keynote. Although a standard Keynote slides looks a bit better than a standard PowerPoint slide, Keynote also has ugly defaults (colours, texture fillings of data charts).