To someone that want to understand it : "It's a currency that is entirely electronic and works thanks to crypto-magic that experts agree is solid. It has the advantage of not being controlled by any central bank and has a guaranteed scarcity."
To someone that just wants to use it : "it charges transfers from and to dollars but unlike paypal charges zero for transactions."
You're assuming that the phrase "crypto-magic" is reassuring instead of off-putting. 'The general public' and 'readers of HN' are basically non-overlapping sets.
My mom has a credit card. Would she use Bitcoins? Not in a million years.
When you use terms like "experts agree is solid" and imply that its very safe with crypto, you, I and other techies will know about the cryptography behind it. However mundanes will think that third is a very reliable system, i.e. something that they can rely on.
When they get stung by a scam on eBay, try to get their money back, and are told there are no chargebacks, that BitCoin is the same as putting some banknotes in an envelope, and that there's no way for the police to track down where the money went (paper envelopes aren't even that bad), that their money is gone with absolutely no recourse, when this happens, they will not think BitCoin is in anyway reliable.
