Currently, most mining power is controlled directly or indirectly by just one organization. This organization has the power to bully smaller miners into voting their way on issues that affect the Bitcoin network's rules, and it's about to become even more powerful:

Bitmain, one of bitcoin’s biggest miners with a current hash-share of around 15% of the network, is to launch a new giant mining facility this December with 45 rooms, independent substations, offices and a total size of 140,000 kilowatts.

By some calculations, that translates to around 45% of the network hash-share, but it is not clear whether the new facilities are simply for a relocation or to accommodate additional mining power.