Pally taunt targets the toon who has threat instead of the mob you'd like to be beating on the pally.
On the other hand, it taunts up to three targets who are interested in eating your shaman while other tanks taunt a single mob.

I use a macro like this for my pally:

#showtooltip
/cast [combat, help] Righteous Defense; [target=targettarget, help, combat]Righteous Defense; Avenger's Shield

Essentially, if you target it on a friendly, it casts the pally taunt on them.
If your target is not a friendly, ie - it is the mob whom your pally is beating on and which your casters are nuking, it casts the taunt on the target's target which is likely a shaman who has over-nuked.
It doesn't matter if you have three shaman's with more threat then your paladin because the mob is pissed at whichever has the most threat, and the taunt is giving your pally more threat then that caster.

You could ditch the combat modifiers and have an exclusive taunt button. I like to cast AS out of combat and taunt in combat on key press.



For a Paladin, this is an invaluable macro:

#showtooltip Hand of Protection
/cast [help] Hand of Protection; [target=targettarget, help]Hand of Protection

Even on taunt immune mobs, you can click this when someone takes threat and make them immune to the mob, which then comes back to smack you (hopefully you're #2 on threat).
Again because of the 2nd part, you can cast this on a hostile and have it re-directed to a friendly.