Often practised by different departments of a company, web design and SEO can sometimes seem like two different separate processes - developers will create websites, with SEOs taking that site and optimising it via content and link-building.
This is, of course, a massive misconception. Good web design and successful SEO are intrinsically tried together - one cannot exist without the other. Poor web design can leave the most dynamic SEO fighting an uphill battle; poor navigation, for instance, means that search engine crawlers won't be able to index pages on your site, whilst duplicate pages will see engines such as Google de-ranking your page regardless of links built. Similarly, an excellent website with bad SEO is doomed to languish away from the front page forever.
One of the most important SEO aspects to consider when designing your website is how easy search engine crawlers will find navigating your pages. The best way to determine this is through running a check on how many pages search engine spiders are indexing when they visit your site.
A quick way to check this is by entering 'site:' followed by your site's URL into Google. This will show all of the pages Google has indexed from your site. However, take the numbers this technique produces with a pinch of salt - the results can fluctuate wildly, sometimes by thousands of pages! A more accurate way to measure page indexing is to use a tool such as Google Analytics.
On the whole, you should know how many pages you want indexing on your site. The number you get from tests won't match this exactly, but if it's massively different, you may have a problem. This suggests that search engines are struggling to navigate your site for reasons that can range from lots of dead links to poor site navigation. Generally, a tool like Analytics will help you pinpoint what's going wrong and how you can fix it.
One aspect search engines tend to look at when generating their rankings is how 'popular' a site is. Links are the benchmark of this, but all links come with varying degrees of power. The best links are one from reputable sources and quality sites producing content on a regular basis. Influencing and encouraging links from these sources is key to any SEO campaign. It's fair to say that a badly-designed site would put off most quality link sources from giving you that crucial recommendation Google looks for.
Consider how someone coming in fresh to your site would act with your site. Get someone who hasn't seen the site before to play with it and note any issues they have with navigation or usability. Similarly, note any design concerns they may have and address them. It's important to remember that, due to the high-value nature of their link, high quality link sources will come up against hundreds of sites a day, most likely clicking off most of them within 30 seconds. By making your site easy-to-use and eye-catching, you encourage lucrative links and make SEO a whole lot easier for your site.