G'day.
I touched upon this subject in the auto-attack thread but this discussion doesn't belong in there.
Current problems with the action bar for gamepads:
Most importantly, it is not pad-friendly but designed to be used with a keyboard instead. For example, you need to scroll between the different abilities to get to the one you want to use. Since even the basic attack has it's own place in the bar, you need to constantly switch between the abilities. Since the bar is one straight line with 10 different skills, it is very cumbersome to scroll. To improve on this, here is my humble suggestion:
Make the bar into a 3x3 grid-style layout:
XXX
XXX
XXX
and make it so the default place of the cursor is in the middle. This would improve the usability of the bar considerably for gamepad users, as the amount of user input would go down by a wide margin. At most, you would have to press the directional button twice to get to the skill you want to use.
However, one skill is missing from the list. That being the basic attack. To solve this, basic attack (assigned by the player) would have it's own button on the pad. If you change the action bar from #1 to #2 or #3, the 10th skill of each of these bars would be triggered with the same button as the basic attack. This way you lose nothing by making the bar behave like this, but it would make playing a lot less of a hassle. Obviously keyboard users could keep the layout as-is.
Lastly, another suggestion regarding the grid: the right analog stick could be used to select the skills by tilting it towards the direction of the skill. This is quite widely used mechanic in other console games with lots of skills to use, and I think it would work here too. The amount of button presses would go down even further.
Thank you for listening. Further improvements or criticism of my feedback regarding the action bar (and party/battle-UI as the whole) for the gamepads is most welcome.