If some rich entity decides to buy all (21 mill) bitcoins, what is the worth afterwards?
Nothing right? A currency is worthless without circulation, so it seems to me a plausible way to attack the currency, if it should prove as disruptive as some people hope.
Even before reaching that extreme, incremental buying and hoarding (or destruction) of coin would reduce the usefulness, and therefore value of bitcoin.
That a bitcoin is worth as much as the cost to produce / mine it is just a proxy value - you can't use a bitcoin to get back that amount of energy / computation power - it's all backwards.

Please enlighten me....

I'm a programmer (C, Java, others) who'd like to contribute if needed.
I also sell promo codes for App Store games for bitcoins.