Scarcity:  There are currently about 12.1 million bitcoins, and as long as the protocol stays consistent, there will only ever be 21 million bitcoins.  In order to change the protocol, it would take more than 50% of the miners to confirm a change to the protocol.   Roughly every 10 minutes, 25 more bitcoins are available in the network. In June 2016 this will change to be 12.5 bitcoin for each block solved by a miner  for the next 4 years, when it will go down to 6.25 bitcoins / 10 minutes in a geometric progression down.