what you are talking now is perceived value.
3 letter and four letter.coms are short and easy to remember..
All domains are worth only what someone is willing to pay for it. So in that sense all domains are worthless, however add traffic, revenue and the domains value will increase...
Consider you owned a stone... its worthless, however if the stone somehow got into the media for being "a worthless stone" that would then create value.
Now.. say.. Apple created a new product called an ipoj then ipoj.com would have inherited value.
Next you have search value.. if a million people searched for the keywords "health cuisine" you would have search keyword value, the will also apply if you had advertisers sponsoring the keywords "health cuisine"
You could also have "Demand Value" where as all the other extensions have been registered and are being used as operational sites, however demand value can sometimes mean threat of Trademark infringement, and can sometimes deflate the value.
Other ways to calculate worth is traffic value, typo value which both are dependant on type-in traffic...
Descriptive Value is another good one.. consider something like BlueRayPlayers.com, NewYorkPhotographers.com.. these domains may have no traffic whatsoever, but they have great potential..
