If you stop to think about it, one domain is no rarer than any other....There can only be one "xyz.com", just as there can only be one "hostingdiscussion.com". What makes one more valuable (or perceived to be more valuable) is how broad of a market--and what market--the name can be applied to. For example, "men.com" recently sold for over a $1M US, but the potential market for a site built around that name is roughly half the population of the world. On the other hand, "tomsbikeshop.com" has a very limited potential market both in the customers of any site developed and in any potential buyers for the name itself. Someone named Frank the Tailor would have no use for "tomsbikeshop.com" but could probably do something with "men.com".
There are several other factors which might affect the value of a name, such as existing site, with or without traffic, how much traffic, reputation of site, links to the site, etc. but most of the discussions seen on boards such as this one involve only a name without a site or traffic so those can often be discounted.
