Pallies and Shammies are all casters.
There's good synergy there.
Every other tank is not a mana user.
Silence can hit the team hard, but that screws over DK's too.

A pally is very easy to macro at 80th.
Go with a 96969 macro, and mash it as fast as the GCD allows.
Consecrate is usable as soon as it ends, and it picks up strays very easily.
Holy Shield is basically up all the time too, and anything that attacks the pally takes holy damage.
My block rate is in the upper 30 percentile range, which is 60 odd with Holy Shield.
There's a 1 in 10 chance any attack procs Redoubt which raises this a further 30 percent.
It's incredibly easy to hold threat with the pally.
All there is to do, is pick things up, worry about facing, and move out of puddles of fire.

Shammies synergize extremely well with both a pally and themselves.
While they don't have fixed buffs like Fort, MotW, AI etc, they have totems.
Four totems per shammy is a lot of buffs.
Shammies have a ton of controlled burst, so when you want something dead, it dies.
For the most part, anything comes into melee and the pally picks it up and holds it.
If they stay ranged, shammies have grounding totems to make caster vs caster a very unfair fight.
Shammies have Tremor which makes the group virtually fear immune, when staggered.

Every member of the team can rez.
Every member of the team can heal.
Every member of the team puts out good dps for their role.
The pally is in plate with a shield.
The shammies are in mail with shields.
Water Shield + Elemental Focus + Mana Spring + Unrelenting Storm + mp5 gear is basically unlimited mana.
Pally with Divine Plea and 2 pts into Guarded by the Light, is similarly unlimited mana.