It completely ruins the feeling of trading, which is really important. For example I only Played diablo 2, because the trading system was awesome, without any global control. In Diablo3 there are well known prices for every item with every stat, and you cannot do a "good bargain" or anything. But if there is no global trading, if you are clever, you can make a lot of wealth.
AND MAKE THE GOLD WORTH A LOT!!!! not like in the other games in which players run around with millions of gold..
Maybe there could be local auction houses in big towns, but not for everything, and every town's auction house would differ, for example different items would be cheaper or more expensive in one city than in another. Maybe for craft materials, etc, but not for unique weapons and armor. Because if thousands of players use the A.H., then there everything will be available for monotonous mouse clicking..