Basically, you will no longer be able to UseAction() or CastSpell*() of any kind in Lua.
In order to enable bar mods to still work, they will be creating a new kind of ActionButton frame. This frame will be associable with a "hard coded" action/unit (for instance, cast the spell Flash Heal(Rank 4)) -- but NOT a Lua script.
While OUT OF COMBAT, you will be able to change what the ActionButton frames do. Once you go INTO COMBAT, the frames will be locked. (This will prevent you from making an ActionButton that represents a unit, and dynamically updates itself in combat with a different spell)
What's even more interesting is that these new ActionButton frames will be *physically* locked while in combat, which means you can't move them around the screen with SetPoint(). This seems deliberately targeted at RDX and other mods that let you choose units from a sort (eg CTRA Emergency Monitor) -- dynamic unit lists will no longer be clickable for spellcasting.