> you say that "They don't require trust from any of the parties involved in the transaction", which makes no sense to me, as the system itself has to be at least partially centrally regulated, both in distributing the original numbers/values (deciding what the requirements are for "earning" money), and in brokering trades.
The only agreement is in the implementation of the protocol, which is open to the public, and if a group doesn't like it they can start a new blockchain with their specifications. Aside from that, there is no trust involved.
The problem is 50%+1 is well known and in fact, against many who argued it would be a problem, the community self-regulated in its infancy by voluntarily switching mining pools. I predicted this would happen, because it would have been against their interest to game the system: when it's all out in the open, you can't cheat. The currency would be worth nothing after that, as people would lose "trust" in the system.
You could argue that the only trust is in the system itself, not in any particular node or authority.
