The maximum number of bitcoins is capped at 21 million. The current number in circulation is around 14 million. Now, my intuition tells me (but I might be wrong) that if a currency is used to pay for goods (and not just for speculation), the value of the currency in circulation should roughly match the total value of the goods that can be exchanged with it.
If this is true, and the total of e-commerce sales is now (completely ballpark estimate) around $1.4trillion, then in a world where all online transactions are made in bitcoins, the value of a single bitcoin should be of approximately $100k.
That means that in the process of a widespread adoption of bitcoin as a real currency for online transactions, its value should rise from the current $200 to $100k, a factor of 500. And this is a huge problem, because any currency that is used for buying goods must have a value that is consistent (and possibly slightly decreasing) in time, otherwise nobody really wants to use it. It's completely foolish to part from a bitcoin in exchange for a phone today, when at some point in the future that same bitcoin will be valued 3, 10, or 500 times more.<p>Bottom line: the bitcoin is trapped. The more goods can be bought with it, the more its value rises, the less people are actually willing to use it. And this cycle will go on forever, as the total value of goods will keep increasing at a high rate, while the number of bitcoins increases at an progressively lower rate until it stops completely at the cap of 21 million.