Hoisted from Comments: Andres complains:
http://delong.typepad.com/sdj/2007/07/mitt-romney-jum.html#comment-76949994: Besides agreeing with most of what is said above, I have to confess that the English language never ceases to frustrate me. Just when I think I have mastered its infinite nuances, along comes a phrase like "jumping the shark" that leaves me in complete confusion.
Compared to the solid certainties of the Romance languages (in which obscure metaphors are generally confined to poetry), English is altogether inscrutable; practically every minute a phrase is born that bears little resemblance to anything that has previously appeared and is therefore fully suited to fool or confuse its readers...
It's a requirement that you have a certain deep pop cultural literacy to understand the metaphors. It is a barrier to entry--a way of giving a certain class a leg up in authority.