Ghash.io has considerable power right now over Bitcoin. During peak periods, Bitcoin has hit its 7 transactions per second limit. If Bitcoin transaction volume continues to grow, it's going to hit that limit regularly. The block size and transaction rate limit cannot be increased unless ghash.io wants it increased. All they have to do is refuse to confirm blocks beyond the current 250KB limit.

Whenever the block chain has a big backlog of unconfirmed transactions, ghash.io is in charge of Bitcoin. They have the power now to effectively impose higher transaction fees, or favor some transactions over others. For example, Coinbase could cut a deal with ghash.io for "express transaction confirmation" - their transactions get processed first. That would be a win for Coinbase - their merchant shopping cart service needs fast confirmations.

The next big issue for Bitcoin may be "net neutrality".