Covering the basics of SEO is important although as I've said before a lot of SEO is just quality control. In a recent post I outlined from an SEO perspective how to optimise a web page but I didn't talk about making sure your visitors don't have to think and ensuring sure you take them by the hand and give them what they need.
We bill ourselves as SEO Web Designers, this means we build sites that search engines can index. But we also make sure the sites we build look after the visitors when they arrive. By this we mean the sites try to make sure the visitor feels comfortable, reassured they have made a good choice when clicking a link to a website and most importantly they believe the site is going to answer their question, solve their problem or sell them what they need. If they get this feeling they may stay on the site, if they don't they'll leave.
Design is a funny subject. One man's meat really is another man's poison. However you can dislike a design but still use it effectively if the design is good. If I hire a car I might not like the look of the car, but if it gets me to where I need to get to, I'm happy.
Given that your design will please some and not please others it's important that it delivers your message/product effectively. Some people will leave quickly purely because they don't like your design but in my view, not many. Lots of people will leave if they land on your page and either can't find what they are looking for or are confused about what they need to do to find what they are looking for. As Steve Krug preaches, don't make them think.