The Idea:
5 spots on a team,
5 caster classes,
5 races

The team:
Demonology Warlock - I wanted a tank if I needed it, so demonology spec to buff the void and make him able to stand up to non-heroic instance boss-level damage: turns out he tanks a lot more often (and a lot better) than I anticipated. Also, crit!
Elemental Shaman - It's the only dps caster spec for shaman.
Shadow Priest - I didn't want a healer, I wanted DPS, so... shadow!
Balance Druid - It's the only dps caster spec for druids.
Frost Mage - I didn't know how much mana I'd be using/regenning, and (it seems to me, at least) fire uses more mana/time. I didn't want to go arcane, because I was not aware of the nochanneling boolean for macros at the time. Frost was relatively mana efficient, the slows were nice, as was the 10% crit from winter's chill, and the mana regen from the water elemental (The mage has aggro issues as it is anyway).

The Cons:
No tank. No big, tough toon to absorb hits and keep my casters casting.
No healer. Not so bad pre-40, but post-40, the mana wasted on getting back into shadowform/boomkin isn't worth the heal, which leaves the shaman and the 'lock the only ones who heal.
Complexity. Each class is different, and needs different keys bound to different spells with different cooldowns and different utilities. Entangling roots? A button all it's own. Water Elemental? That's on it's own button.
Different damage types. When you fight a mob with some sort of magic resistance/immunity, chances are at least one of the toons is going to be doing suboptimal dps.

The Pros:
Buffs. With four totems, Prayer of Fortitude, Gift of the Wild, Arcane Brilliance, Moonkin Aura, Prayer of Shadow Protection, I'm as buffed as I'm probably going to get - short of a raid - and most of the missing buffs aren't really for casters. Also, because of the varying types of buffs on this team, there is little to no redundancy: ie, buffs that don't stack with each other.
Crowd Control. Polymorph -> humanoids and beasts. Hibernate -> beasts and dragonkin. Shackle -> undead. Banish -> demons and elementals. Fear -> everything but undead.
DPS. With no tank and no healer, things die fast.
Mana regen. Mana spring totem + Vampiric Touch + Improved Water Elemental = never needing to drink.
Food and Water. Unlimited free food and water from the mage.
Transportation. Portals from the mage, summons from the 'lock.
Quick wipe recovery. Soul stone and reincarnation means there's almost always someone who can get back up again, and three rezzers means that the entire team gets back up faster.
Different damage types. I shudder to think of doing ST with all shammies, what with the green dragonkin being resistant to nature, and the five dragonkin bosses being totally immune. If you do run up against a mob who is resistant/immune to a specific damage type, you've still got the rest of the team who can dps.
Healing. Although the boomkin/shadow priest would go through a lot of mana to get back into their dps forms, they can heal if need be, and help the shaman out.
Options. If at some point this team can no longer manage to do instances, the druid can always respec feral tank, and either the shammie or the priest can respec to heal if need be.

Leveled off of quests to 14, then hit instances, and almost never looked back. The only times I did quests outside of instances, was Jintha'alor, HP quests to get some gear so I could do OL instances, the Winterspring quest chain that ends in killing High Chief Winterfall and getting the Spell Power trinket, and quest chains that led up to instance quests that I wanted to do.

(current) DPS Button (@64)
Shadow Bolt/Demonic Empowerment (random)
Frostbolt/Deep Freeze (random)
Wrath
Lightning Bolt
Mind Blast/Mind Flay/Mind Flay (sequence)

Boss DPS Button
Shadow Bolt/Demonic Empowerment (random)
Frostbolt/Water Elemental/Deep Freeze (random)
Starfire
Lightning Bolt/Chain Lightning (random)
VT/MB/MF/MF (sequence)

Instant/DoT button 1
Corruption
VE
Flame Shock
Moonfire
Fire Blast

Instant/DoT button 2
Curse of Agony
SW: P
Insect Swarm

Most fights start by making sure all buffs are up. Then, set CC targets, send the void in, set off the CC, and dps down the targets one by one. If the void starts taking too much damage, I'll switch the 'lock to health funneling, and maybe toss a quick heal from the shaman. I don't bother casting DoTs on trash fights, things die too fast.

Bosses, send in the void, give him a couple seconds to build up aggro (with Demonic Empowerment), drop Treants, throw up DoTs and other instant spells, then dps him down. Most bosses don't usually last long enough for the DoTs to wear out, or if they do, they're not down for long before the boss falls. If the void takes too much damage, once again switch to health funnel/healing wave.