Having an easy to use, well thought out website design is important because you would like your potential clients to stay on your website and not get overwhelmed or find a reason to navigate away. Your content should be easy to read and on topic, making the right choices in your design will ultimately decide how successfull your website is.
A good website needs to be responsive. This means it needs to work well and scale across all types of devices being used today. It should be a single instance but Intelligent enough to know how to resize itself based on the device it is being viewed on be it be a computer, tablet or mobile phone.