I see job listings that list Design, UX and SEO separately–but increasingly they are the same thing. With the changes that Google has made to their search engine, it is no longer based on keyword hits–it is based on user’s experience–the ability for them and popular websites to link you as a premiere place to be on your keywords.
The 90′s are gone–relying on keywords in content alone is increasingly becoming less and less important. What matters is that you have a design that communicates to the base user the message you want.
Also, it means you can no longer stay static–you have to take risks and change with the markets–having the same design without upgrades for five plus years is no longer an option–your ranking will fall.
You can see my point proven in http://www.copyblogger.com/seo-site-quality/#more-22978. Also you can test your own website at Alexa.com where you will see the majority of the weight put into who links you.
Now how can you improve user experience? You do it through clean design and structure. Design, unlike for the artistically challenged, does have a psychological basis–this is what people call user expectation. When you break it you need a good reason for it–when you meet it it makes your user happy. When you make for meeting user expectation again and again and even exceed it, they will link your website–or at the very least bookmark it as a go-to.
A good design is not just fancy colors and slapped on text, it gives mood, thought, feelings, message, even without the words. It is just as important as getting the backend clean–because it is your primary communication tool. It is User Experience. It is SEO.