I would very much love to see two buttons on or near the action bar. One to toggle AOE for offensive spells and the other to toggle AOE for friendly spells.
Even one button for both would be acceptable. It can be pressed or simply used as a visual indicator for those who would just like to press Z, for example, to toggle it on or off. While toggled on, all spells would be AOE by default; whereas if it were off, all spells would be single target only.
The removal of that prompt after every single spell would greatly improve the enjoyability of playing the DOM classes, for me anyway.
I imagine the current implementation was probably just a temporary solution and something similar may already be in the works, but I thought I'd get this particular discussion going since it doesn't appear to have been brought up in its own thread recently.