One of the problems that drives me crazy is when you pick up many stackable things fast, you end up having 2 here, 1 there etc in your bag.
What i purpose is this,
Having it where you can hold down ctrl and left click on the item in your bag, and it will auto stack those items that is in your bag, into the needed stacks. So you don't have to spend 20 min stacking everything up nice and tidy. I have seen this option in a few other games and found it to be very helpful, especially when your in a hurry.