This discussion site we linked the other day, Epic Advice, is humming with activity, and full of interesting queries and answers about the World of Warcraft. Like this one: which city is the best for running to and from the bank and the AH (the question-asker also wants to get the mage portal in that circuit, but for the purposes of our discussion, we'll assume you're on a bank alt, so no portal necessary)? In terms of speed and ease, which is the best capital city to set up shop in?
On Horde side, the answers suggest Orgrimmar, and I'd actually argue that one -- Thunder Bluff is my city of choice for pure Auction House action, as it's a short walk back and forth on the Lower Rise from the AH to a mailbox (and as other comments on EA say, the forge is right there as well). UC is definitely a mess, as you have to go up and down those ramps, and Org is close, but it's so busy that it's almost distracting.
For the Alliance, Ironforge is an old standby, though I have to say that Stormwind has its advantages. You do have to wind between buildings a bit to make it back to the AH from the mailbox, and it is busy (almost surprisingly so), but it's a little easier than Lagforge, and once you know the route, it's fast. Can't say I've ever bothered with the AH in Darnassus, just because getting out there is such a pain. What do you think?