Increasing difficulty of mining Bitcoin
With mining Bitcoins getting progressively more time consuming as the difficulty of finding them is logarithmic
see this difficulty chart: https://blockchain.info/charts/difficulty
it means there will (soon?) be a tipping point where it costs more money in electricity to mine them on a mass scale than it returns in Bitcoin value. That's when things could get interesting, with more people dropping out due to the unprofitability, will the big players dominate the market and achieve 33%?