What % of HN knows what this means? Maybe this should be a blog post instead and explain what a "mining pool" is. Or what is the significance of one pool taking over the market.
Anyone familiar with Bitcoin is likely aware of the 51% attack. A blog post is hardly warranted, because this isn't significant. GHash.io does not control all of that compute power. It is a metalayer that coordinates distinct and unrelated compute resources controlled by (supposedly) 180,000 seperate entities[0]. In order for a 50% attack to take place the vast majority (at this point) would have to agree to attack, and that simply won't happen.
I don't think you're right. The mining machines are pointed towards Ghash pool owner and he becomes the owner of the mined block - that's how he can distribute the BTC to everyone who contributed. So in practice, he controls the whole hashing power and can perform the attack without users' consent.
[0] GHash.io homepage