One of the biggest innovations Bitcoin brings to the world is a way to conduct commerce without the need to trust other human beings.  By passing 50% (one could argue even 40% is too much), ghash.io changes this equation and now requires that we trust the person(s) running them -- whoever they are.  Apologists for ghash.io claim that it is not in their best interest to do anything to compromise the network, but it does not take much imagination to see that there are many scenarios in which this supposed conflict of interest does not prevent malicious mischief from taking place.  This flies in the face of everything that Bitcoin once stood for.

I believe I read recently that some of the core developers (including Gavin) are of the belief that there are no proactive measures they can take (as developers) to effectively stop a 51% attack and must instead wait until it occurs before introducing defensive changes to the code.  While there may be many technical reasons which support such a viewpoint, this should not make anyone who is s stakeholder in Bitcoin feel confident.  One can only imagine the selling frenzy and resulting price drop when the first significant double spends are confirmed.  Bitcoin has enjoyed a first mover advantage since the first altcoins appeared on the scene many years ago, but this is exactly the sort of thing that could erode its lead.

There are other ways to mitigate the problem.  One that has been already tried but appears to be ineffective is to appeal to the community to move their mining power to other pools.  Certainly there are other measures which I haven't seen discussed yet.  In the past, I have seen pools, even large ones, shut down for days at a time by large DDOS attacks.  Perhaps watching their mining revenue drop to near-zero for a couple of days might influence the owners of ghash.io to modify their ways.  Billions of dollars of market value are potentially at stake here compared with a much smaller loss in revenue for ghash.io of they are forced to limit themselves to less than 40% of mining.