It's the number of keys you need to click, for the toon to tank effectively.
A single boxed -insert tanking class- can easily hold threat in any situation.

When you're also clicking the heals for the healer and the dps for the damage dealers...
Your ability to click a lot of abilities on the tank go down.
The more classes you bring into the mix, the more options/buffs your team has, but the less efficient each class is played.

A paladin can get by with:
/castrandom Consecrate, Holy Shield, Hammer of the Righteous

You could put that macro on keys, 1-4 and 7-0.
Assign keys 1-4 as dps keys.
Assign keys 7-0 as heal keys.
Anytime you're DPSing or Healing, the paladin is clicking a tank/threat move.

I won't claim that it is an optimally played paladin.
But it is playable, and you can tweak it from there to add additional functionality.
Don't think DK's, Warriors or Feral Bears are as easy to multibox, even if they're as effective tanks.



*Edit*
Looking at your groups, you have:

1 - Paladin (Tank Plate)
1 - Hunter (Mail) x3
1 - Priest (Heal Cloth)

2 - Death Knight (Tank Plate)
2 - Mage (DPS Cloth) x2
2 - Warlock (DPS Cloth) x1
2 - Paladin (Heal Plate) x1

As far as gear is divided, both the DK and Pally tank will want the same stuff.
The 3 hunters will each want the same things.
The 2 mages and warlock will want the same things, priest might want some of their stuff.
Pally healer is gets basically whatever they want, aside from trinkets.

The Pallies get both tanks Blessing of Sanctuary and Kings. Others get Kings + Might or Kings + Wisdom.
You have Fortitude and Arcane Intellect, Mage Food and a Soul Stone.
If any of the hunters are engineers, you might have Jumper Cables.

I'd consider a Druid, both for MotW and because Caster Leather will rot.
An elemental shaman seriously buffs a caster group, brings Heroism and no one else cares about Caster Mail.