What bothers me is that the arbitrary total of 21 million BTC is far less than the total number of human beings alive today. This is far less then 1 BTC per person, at a certain point any transaction in BTC will be completed in fractional values so small as to make the currency completely unwieldy. This is despite my belief that an alternate currency by choice sounds like a great idea.
Each bitcoin is divisible down to 8 decimal places. There is nothing "unwieldy" about using these fractional values, you just use different units, eg 1 mBTC = 0.001 BTC, 1 µBTC = 0.000001 BTC, etc. It's not like we have to actually mint really tiny coins or anything. 