Frinka over at Warcraft St listed some features she would like to see added to the Auction House. My favorite is what she calls Offers or Bounties. Basil Berntsen calls these Buy Orders. Buyers would post an item they would like to purchase with the price they would like to pay. This way, buyers wouldn't have to keep searching the AH for the things they need to show up, and sellers could reduce the relisting of items.
I think that a bid option would enhance the system. The buyer would post a preferred price, but there would be an ability to bid as well. Giving the sellers the ability to make an offer would make the Buy Orders used more often.
Yes, you can get in trade chat and say "WTB [insert item here]" -- but, you know ... trade chat. Also, the buyer and seller have to be online at the same time for it to work. Trade chat offers the benefit of no AH fee but otherwise has too many obstacles to be very useful.
What changes would you like made to the AH?