I like Josef Assad's explanation of SMW in place of the "big load of do-not-care" on its Wikipedia page :
Semantic Mediawiki allows you to attach properties to wiki pages. And to
query the pages based on these properties.
It doesn't quite capture the elephant (e.g. you add these properties with wiki markup around existing links and values on the page), but it's short and sweet.
How soon will this EAR equivalent come in a cardboard box to "sell for amounts of money so vast that the base license includes the very souls of [at least one] fairly recent management school grads"?