Don't wait - getting traffic, getting indexed in the search engines, getting links etc takes time, so the earlier you go public the quicker all this will start happening.
You can change the design as you go and but it's the quality of your content that will win readers over. Also, consider that later on when you change the design to something better you will have a positive impact on the audience you already have - sometimes it's smart to start with something you can improve for the sake of impressing people when you make the improvements...if you get my drift!
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