The keyword within your domain name is so important. Search any keywords in a live domain name, you will find that the domain will be at the top organic search result (or very close).
The domain name is also important as this is what people see. If you are a Professional Golf Shop and you have the domain www.happyfishing.com people will be unlikely to understand what you do and less likely to click through to your website.
The advice really comes round to choosing your company name (as this should be the name of your domain preferably). You should have maximum three words in your business name/domain name. The first should be something that stands out, i.e. blue. The second and third words should be what you do, i.e. tennis balls. This would be great for a company that makes Blue Tennis Balls. www.bluetennisballs.com. At least everyone now knows what you are doing even if it is a strange idea!