I had an idea yesterday when a random guy gave me 500 gold just for rolling my first Alliance toon. Problem is, I can never seem to get an Ally to around 20 before losing interest and deleting, plus my Horde toons could benefit more from that cashola. Then the idea came to me... all gold from all characters on your account is accumulated into one big gold pool that all your characters can use. They would have to get rid of the gold cap of course, but I think this is a pretty good idea because it saves time from sending gold to alts and such. Maybe you see something on the Auction house for a very low price and want to buy it before someone else! No need to go back and forth to the mailbox, all your gold is already with you!
Why stop there? Why not account wide currency? Honor, Justice, Valor, the whole shablang!
I honestly can't see any problems with this simplw idea, except mailbox dancers may lose a lot of revenue because people aren't visiting mailboxes as often... It could also be hard to code too, no idea...