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One of the things that I always like to do in mmos is to play the auction house to try to accumulate as much currency as I can. Imagine my delight to recently find out that the GW2 auction house will be global over all the servers. Awesome !
Am mmo economy will always benefit from the number of players that it gets and in normal mmos if you end up on a low population server (not that I suspect that will be a problem in GW2) then the economy tends to be poor with not a lot of activity on the AH. A global economy will be just brilliant.
You will also be able to transfer currency from a toon on one server to one on a different one by placing an item on the AH for a stupidly large price and buying it with the rich toon. OK, you will loose whatever commission is charged but that not too bad. Many people use this method already to transfer funds between toons in different factions in WOW or Rift for example.
I also very much like the that you will be able to place orders on the AH for other people to fulfill. I'm not sure how the game is going to enforce orders are going to be honored though. Would be a bit annoying if you make a rare item only to find that the person who ordered it then didn't want it anymore.
Roll on release date so I can start the long haul to become the games 1st millionaire !!!