the number of bitcoins is virtually limited to 21 millions, we've got less than 1 million users for now but imagine one day the price of the bitcoins becomes so high that a transaction fee will cost several bucks.
the minimum transaction fee is 0.0005 btc thats currently 0,2 cents per transaction...
that's nothing alright but this is only the beginning, due to its strict monetary policy, the bitcoin will kill itself when the transaction cost will be very high this will refrain a lot of people from doing transactions.