The gold standard simplifies things in one way but complicates them in another. At the moment, from what I understand of economics, most nations have floated currency based on how people feel about the nation, whether it can pay debts, its GDP, long term prospects and the like. Our money is like stock in the nation, not always based on reality but broadly speaking linked in some way to our well being. If WE went to gold, a lot of nations might do the same and then it's a fight over who has the most gold to back their currency - and whether gold is worth anything. It ties the national and possibly world economy to one commodity. In addition, wouldn't that mean that nations like South Africa, who have a lot of gold still to be mined, may end up being major powers simply because they have so much gold? GPEC (gold producing and exporting nations) would all be powers like OPEC is now.
I'm with Vararam, too. There's a good chance a global currency may form not by design like the Euro but by necessity and evolution in a PayPal sort of format or like Square.