the above poster gave very good advice.

if you think of your team as a unit, max out the skills that's useful for the team and not just for themselves.
For the pala you'll probably use the most abilities.
mages will probably shoot fireballs and do some arcane explosions? simple.
priest heals.
shaman toss some supporting totems and nuke.

that's an awesome setup. but also give each character a backup actionbar when they're not targetting the main so they can do some damage on their own (in case someone dies and you have to control one of the slaves)


Priest:
i'd give my priest healing commands based on who im targeting with macros.
basically, if the pally targets a harful mob, heal the pally.
of the pally targets a helpful mob, party member, or passerby, heal the pally's target.
i would change spells based on modifiers (alt, ctrl and shift).

Mage:
1 button for arcane blast.
1 button for frostbolt/fireball.
cant think of anything else mages will actually need.

Shaman:
1 button to throwdown totems in any order you wish.
1 button to nuke along with the mages.

so with the three classes above, we've used up 5 buttons at most. that leaves at least 7 for the pally with additional clickable buttons. also dont forget to make one for drinking/eating and mounting.