Regular currency is legally enforced by governments as the way to pay taxes, and settle debts in that currency - ergo you always need some or need to buying some, even if you don't want to deal in that currency day-to-day.
BTC does not have these things. The only thing holding it up is irrational speculation. Every person who holds bitcoin needs to sell it to a greater fool for a currency which does extinguish debts/tax obligations locally.
Consider: a population of people in the US dealing solely in BTC, and earning income, have to pay income tax. The IRS can assess income taxes as the relevant fraction of their income (easy enough: assess an approximate USD market wage that they have worked, declare they have to pay USD % of that in taxes). The effect would be BTC craters in North America, because every member of that population would have to somehow sell BTC for USD - only none of them hold USD, so they can't trade amongst each other, which means they need to convince someone else to accept BTC for USD (and I mean, why would you? You can't pay taxes with it which is why they're selling)?
For example, any real estate with a premium for its "better location," suits by Armani, sunglasses by Gucci carries a similar premium. This applies to gold as well--a huge part of the value is the expectation that others will continue to like shiny things.
Nothing anchors their price, or premium, other than a system of social beliefs and expectations. Specifically, beliefs about how others will perceive them are all crucial in how people value these items.
The same is true for bitcoin. It starts with a belief that others will also prefer to store value in currency with a higher utility--storable with no fees, and instantly transmissible lower fees--lower friction. It isn't so much the greater fool theory as it is the starting of a journey toward a Nash Equilibrium. But first everyone has to come to a belief about how they, and others view the currency. http://en.wikipedia.org/wiki/Nash_equilibrium
Once this has happened and additionally the markets have become more liquid, things can level out. Right now, the world-brain is confused about how it feels about bitcoin.
