There were some complaints that the large monster hitboxes in the Diablo 3 beta could make it more difficult to run away, as you would sometimes click a monster hitbox when you only wanted to click the ground, causing you to attack instead of move.
An "always move" key was suggested on the European battle.net forums, and a Blizzard CM said that they may actually implement the feature, albeit in a patch after the game's release:
However, there have been some more complaints about the large monster hitboxes, namely that it's difficult for a player to target an exact area in the middle of a monster pack without accidentally targeting one of them.
So, this suggestion is just an addition to the "always move" key suggestion - I think the "always move" key should also ignore targets for target area spells.
"Ignore targets" key
Holding down the "ignore targets" key (could be the tab key) would ignore all monsters or boxes on the ground or whatever you put your cursor over; monsters that you hover over would not be highlighted in red, and they would not be targeted when you click them while holding down this key.
This key would have 2 uses:
1) Always move
This is mostly useful for ranged classes, because sometimes if all they want to do is move and they accidentally click a monster, they will cast instead of move. Holding down the ignore targets key would make LMB clicks always trigger a move command, regardless of what your cursor is hovering over.
2) Cast to target area only
Sometimes you want to target just an area, and instead you accidentally hover your mouse over a monster, so you cast directly at the center of that monster, which is a slight distance from the actual area you wanted to cast to, thus hitting fewer monsters. Holding down the ignore targets key would enable you to cast a spell to the area directly under your cursor, not at the center of any monster whose hitbox may be under your cursor. This would work on any targeted spell that is on the hotbar or in the RMB slot, since LMB would only be a move command when holding down the ignore targets key.