As long as you can accept the fact that you won't be playing any class optimally, running multiple classes is not that bad for levelling purposes. Things will change up and become more complicated when I figure out which heroic dungeons I'm going to box daily...

This is what I did w/ the A-team

1 = Assist focus
2 = Send in pets and start melee attack (Paladin)
3 = Instant cast spells (for ganking when there's competition)
4 = Hunter-Steady Shot, Mage-Fireball, Priest-Smite, Warlock-Shadow Bolt
6 = Ground mount
8 = Priest does Greater Healer on focus (changed to paladin when I was truly tanking as opposed to just mass DPS)
9 = Eat food/water
0 = Flying mount
- = Reserved for quest specific items (some quests give you multiple credit if you do them all at the same time, such as converting the sentries on the isle daily)
= = Follow focus

I mainly controlled my lock and had healbot so it was easy enough to mouse over and shift-click or ctrl-click on the appropriate mob to heal.

With that setup, all I really had to do was... get near mob, tab, 1, 2, 4, 4, tab, 1, 2, 4, 4, tab 1, 2, 4, 4.... loot loot loot

So... easy for levelling.. the challenge comes in dungeons where you can't afford playing sub-optimally and need to use more than one spell.