Beta testers may have noticed that the basic attack is no longer assignable to a mouse button. This is because it’s not in the skill interface to select. All characters begin with a skill on the left and a basic attack on the right (melee for Barb/Monk/WD, ranged for DH/Wiz) but once you’ve swapped that out for a skill you can’t get it back. You still go to basic if you’ve run out of resource to use a skill. Khalah wanted to determine if this is a permanent feature:
I hope that it did not make it into the patch as an oversight. I like to do unusual things like play melee demon hunters/wizards (sometimes) and also when fighting the skeleton king it would be nice to be able to pop a few guys with my sword while I regenerate mana on my witchdoctor.
Blizzard clarified the situation:
In later builds, we’re looking at having Basic Attack as an option under “Primary” in the skill UI, which will (by default) assign it to the left mouse button. Also, as many players have already noticed, your hero will default to Basic Attack if there aren’t sufficient resources to power a given spell or ability.
Just so that there aren’t any misunderstandings, we are still evaluating Basic Attack and haven’t yet decided on a best course of action. It’s our intent that there is a way to use Basic Attack, but our exact plans may change between now and launch, or even between now and whenever we next patch. This is where we’re at currently, though.