There are going to be a bunch of people who disagree with this statement without reading the reasons here and too bad for them.
When building a website worry about content first, design second.
Now there are certainly some obvious exceptions to this rule, but unless you plan to get all your traffic through a ton of work spamming forums and manually creating links, you have to initially rely on search engine traffic.
Can you run a good website without search engine traffic? Sure you can. Some people do. But I wouldn’t suggest it starting out. Getting traffic without being listed in Google is possible if you have an established follower base or have a strong offline presence, but if you’re reading this it’s likely not the case for you.
So, why content first? Because even with a completely bland design, search engines will find your content and people will still read it. GoogleBot doesn’t care if you have a background photo or nice title images on your widgets or rounded corners on your images. Nor do your readers that use a content aggregation program like Google Reader, they’ll never see it.
Each day your content isn’t backlinked and indexed in the major search engines is a day wasted. Content age and link age have a great deal to do with your ranking. Worry about content first.
I am creating a number of brand new sites for a link building experiment I’m announcing next week, and I found myself spending an hour yesterday looking for and photoshopping together just the right background image for it. BEFORE I’d written a shred of content for the site, BEFORE I’d created a single link to it, BEFORE I had any traffic at all.
Then I kicked myself. Content first. Design second. Once the site it getting traffic, I’ll worry about prettying it up to keep visitors engaged.
