The Ethereum fork will take place in a matter of days. I recently skimmed through it, so here are the kinds of issues that I worry about with regard to the hard fork to come, as well as half-baked ideas related to the decentralized administration of the hard fork. This is a brain dump of sorts, so treat it as such. Not every potential problem turns into a real problem. I will scientifically lay everything I worry about on the table.
http://hackingdistributed.com/2016/07/19/how-ethereum-hard-fork-can-fail/