I'm not sure -- I haven't used Keynote!
I've just noticed that people using Keynote's slides simply look better than mine. Every time.
Probably subtle differences in default whitespacing, fonts, etc?
There are a lot of things you can do, and a lot of dirty tricks (possibly my favorite is using low-opacity rectangles to increase contrast between foreground text and a background image). Zach Holman's design guide is an invaluable resource.
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