1. The limit on the total of Bitcoins that may ever exist is, numerically speaking, much too small compared to the world money supply, which leads to a significant psychological barrier - since it is very difficult for a normal "man on the street" to believe that 1 BTC could someday be worth $100,000 or more (as it would have to be if Bitcoins ever became as widely adopted as the US dollar, say).
A dollar is divisible to 2 decimal places ($0.01). A bitcoin is divisible to eight decimal places (0.00000001). If you express the total number of bitcoins to two decimal places, you get 21,000,000,000,000.00 (21 trillion).
+1
When considered correctly, there are actually 2.1 quadrillion "bitcoin units." If Bitcoins rise in price to such an extent that we cannot efficiently purchase a coffee with the smallest such unit, then we can all relax, because the currency already succeeded in taking over the world.
We say there are "21 million" just as a notational issue. But each of the 21 million pieces is a group of 1 million smaller units.
Yes, but I think it's a serious problem that the entire Bitcoin community has been talking in terms of the larger unit since inception. If we had just been using the smaller units all along, the BTC price might not have attracted so many attacks, and might still be growing steadily and quietly.