The Ideal 5-Man group Make-up

I love this group make-up for 5-man instnaces and here's why:

Warrior Tank
Warriors were made to tank. It was an afterthought that possibly paladins and druids should be able to tank too. If you had to swap someone, you could change out the warrior for a paladin tank, if they were awesome. That way, the pally could hand out Kings or Wisdom to the party which would all benifit immensely.

Priest Heals
The priest is the only true-blood healer in the game. There are very few things that a resto shaman, resto druid, or holy paladin can offer that a priest can't. While druid heals offer more HoT's, the priest's Prayer of mending and renew are usually a much more efficient use of mana. Throw in Fortitude and Spirit which would be immensely useful for all other players in the party.

Mage
Not to mention the heavy DPS, polymorph is probably the best CC in the game. Arcane intellect would be beneficial to every other person in the party. The mage has the most consistant AOE damage of any class.

Shaman
Totems are freaking awesome. Tremmor totem to mass-dispel fear, mana regen, windfury, and a fire nova totem for AOE. I'd prefer enhancement here as the DPS is just amazing.

Druid Moonkin
Throw in a battle rez, innervate, and a 5% crit aura. Very nice spike damage and AOE capability here as well.

It also helps that having a pally, priest, shaman and a druid means that you have 4 people who can throw out an extra heal / rez if needed. If you consider their common off-spec, this is a pretty versitile group if things needed to be switched around in a pinch.

You could easily have:

Druid Tank
Pally DPS
Shammy Healer
Mage DPS
Priest DPS