And who is the UN accountable to?
As we're seeing with the Oil for Food scandal, the UN is unaccountable to anyone with widespread corruption from the top on down. Congress is discussing the idea of cutting our funding (some 22% of the UN's operating budget), but other than that, nothing is likely to happen.
Extra-national courts are an idea whose time has not yet come. Even the post WWII war crimes courts had a pretty checkered past, especially in the Pacific.
If courts without accountability are such a good idea, why bother to elect legislatures? Just let unelected and unaccountable courts make all the laws as they see fit.