I find the pally with a 96969 type macro gains aggro automatically.
They're incredibly easy to box with.
Basically you start with full mana, which is the equivalent to a full rage bar on a warrior/druid or full rune power on a DK.
If you have the talent for Divine Plea to refresh on a melee hit, your mana does not drain much at all.

Granted, I'm boxing one of the easiest combinations.
Prot Pally and 4x Elemental Shaman or 3x Elemental and 1x Resto when necessary.

Personally I'm micromanaging my shammies, with four dps keys.
Lighting, Chain Lightning, Flame Shock and Lava Burst.

My pally has the 96969 macro on all four of these keys.
I've not been able to take aggro from her yet, with the exception of my full Naxx80 shammy vs a lower level Pally.
If I open with anything other then Flame Shock, or give the pally a 2 second start...
The 80 full naxx25 shammy has not been able to steal aggro from the pally since the pally reached 77, in quest blues/greens.

Threat isn't really much of an issue for most tank types in Wrath.

Even with a mixed group, you could via the click castsequences have fairly effective DPS on one mashable key.
I don't think it would be much different with a mixed group or a 10-box.
Pallies rock as tanks.