I don't think we've yet discussed very simple explanation for the exponential price rise: supply and demand (particularly supply)

So, supply and demand, the most basic way to determine the price of traded goods.

As we all know, demand for purchasing Bitcoins has been growing rapidly over the course of this year - speculators, mostly, but also users interested in conducting transactions in Bitcoin (Silk Road and Atlantis, for example). However, we can't know how rapidly it's growing - how many people, after learning about it, jump in to buy?

However, there's a big and painful issue of supply (and I'm not talking about Bitcoin's halving of block reward). I define supply as the relative ease with which I can buy a bitcoin if I have the funds today. Put it simply, it's a huge pain in the ass to get your dollars/euros/pounds converted into these shiny cryptocoins.

Just consider this:

• No quick way of buying Bitcoin - getting money into exchange takes days and is far from straightforward
• Unreliable, fragmented exchanges - including hacked (Mtgox, Bitinstant) and downright malicious (Crypto X Change)
• Fraud, scams - multiple reports of users being scammed when buying and using bitcoins
• Sellers unwilling to sell - multiple reports of how difficult it is to buy large quantities of bitcoins. In fact, this is why $100,000 buy order makes the price shoot up 5-10%.
• Bad press - uneducated journalists doing half-ass job and/or maliciously spreading misinformation

Therefore, I consider supply (as defined by ease of purchasing bitcoins) to be artificially limited today, which contributes to the rapid rise of price. Perhaps the supply issue is the primary cause of such rapid rise of price! As bitcoins become easier to obtain and demand growth subsides, we should put this period of unstable "rollercoaster" growth behind us.