Since the days of D&D tabletop, positioning (travel order) has been important for the beginning of tactical combat. As it is, the AI code for auto-positioning does a poor job of deciding who gets placed on the front line, and who is blocking whom, especially when your armies get larger. I often find my warrior/defender/assassin champs stuck behind my ranged groups. Heh, no (sane) ranged unit would volunteer to stand in front of their protection when going into battle (especially when picking a fight with frothing-at-the-mouth beasties with charge).
I realize there are simply too many fights to add a positioning phase on the tactical map where players could set their own units (within a small area equal to the number of units in their army). But instead of tweaking the AI, perhaps you could let players set their placement order before entering combat, like on that pop up confirmation screen that shows what enemies you are going up against (so the default positioning that the system has to work with--unless changed by players--would cost no extra clicks). This would be a nice nod to the olden days of the DM asking the players what their travel order is, and the game could simply place the first 3 units on the first row, the second set on the second row, and the remaining ones on the back row, rather than trying to figure out if that champion is a charge-forward guy or a hang-back guy.