Domain names unrelated to your topic can easily deter potential readers from clicking on your site. As an example, consider a situation where you may decide to invest in an AdSense campaign, or paid search engine placement. Irregardless of where you advertise your site, the domain name is likely to be visible to the reader. If it is unrelated to what they are looking for, chances are they will not click on your site.
Unfortunately, in some cases, the main keyword for your topic may already be included in a number of other domain names. In these instances, you may want to us a domain name that still includes your main keyword, and also a secondary one. That said, it is important to keep your domain name to a comfortable word length, so that it is easy to remember.
Finally, even though hyphens in a domain name may make it possible to use your main topic keywords, they can be very annoying to your potential reader. For various reasons, a potential site viewer may want to type the domain name into the address bar. If you have hyphens in the name, it is easier to mis-type it, and then receive an error message. In all cases, it is important to realize that web surfers rarely have the patience to wade through long domain names, let alone figure out why the error occurred.
Many people want domain names that are fresh and live. Unfortunately, even if the name is very creative, it may not be related to your product or site topic. Aside from this, long or tedious domain names can inhibit potential viewers from returning to your site at a later time.