It is hard to nail down why good design looks good, and poor design does not. We see it, but articulating why, it is hard. A good designer has an "eye" for design (a combination of talent and experience) to make things just look right.
Having said that, here are some of the things to pay attention to.
- Layout along a grid, everything is spaced out, lines up
- Colours, not too many
- White space, have the courage to leave things blank
- Font sizes, maybe smaller is better than screaming big, if it adds white space to the page
- Limited number of words per line (10 points text on a 16:9 slide spanning the entire width does not read very well) and carefully selected line breaks
- Nice images that set the right mood (avoid cliche stock images)
- Use vertical positioning as well as horizontal ones (a different way of saying there are more slide compositions than lists)
- People thought about every word, sentence, shape on the page. Changed it, repositioned it. Changed it again. Good slides very rarely come out right in the first iteration.
All the above can be found in any design book. We read it, we understand it, and still when opening a PowerPoint template when we are back at work, we ignore pretty much all the rules.
A good exercise could be to make one slide (just one), look exactly like a poster designed by a Swiss graphic designer from the 1960s that you like, or maybe like a slide that Steve Jobs used once. Push yourself really hard until the style is identical. You will be surprised by the amount of changes you need to apply to your design.
And hey, my presentation design app SlideMagic makes sure you stay on the right path. Try it out.
Art: Georges Croegaert - The Philatelist