Often when I produce a slide with simple rectangular boxes and just once accent colour plus a black and white image (hey that looks like a SlideMagic slide), I get the comment that "things look really simple, unsophisticated".
No icons, no shadings, Helvetica, no drop shadows, no rounded corners, no gradients, no nothing.
Here is the trick: it is the composition of the slide that makes things sophisticated. And that is the hard part to get right. Look at the work of the famous Swiss graphics designers of the 1960s. Most of them designed posters with the very same tools that you have in your hands when opening PowerPoint.
Look some of the simpler posters, look at your slide, look back at the poster, look at your slide. Spot the difference, and fix it!. It is layout, not fancy graphics.
And, my presentation app SlideMagic makes it a bit easier than PowerPoint or Keynote.