When I first heard about bitcoins, my biggest concern was that the system stops creating BTC supply at 21 million.  This implies future deflation because prices are determined by the supply and demand of a currency.   If supply of the currency is flat while demand is growing, you get a massive deflation.  Deflation means that your BTC will buy more next year than it does this year.  For example, this means that a bitcoin today can buy one apple but next year can buy two apples. 

Deflation is toxic to an economy because it promotes people to save more/invest less.  This creates a downward spiral because when people save more/invest less, there is no money flow to economic growth.

This is why the Fed is so actively printing money with QE etc. to prevent a deflationary spiral for the US dollar.

In BTC's case, the only long term saving grace is if it can establish the fractional reserve system.  This would allow regulated entities (banks) to lend BTC to each other and continue increasing the BTC supply