To me, Retribution Aura seems to be the biggest misnomer in WoW, and is additionaly in the wrong talent tree.
As a Retribution Paladin, you are almost never going to be using this Aura. At the lower levels when you CAN tank stuff as a Ret, sure you are going to use it if you are the main tank in a 5-man like Uldaman or something like that, but that is about it. I don't know of ANY Retribution Paladins that put talent points into improving this talent, because once you have Sanctity Aura, Retribution Aura is completely off your list of Auras to use.
I have limited experience playing a Protection Paladin (hope to have more soon!), but it seems like Protection Paladins would use this Aura a LOT... it provides quite a bit of additional passive holy damage, and when you are tanking large groups, it just provides that much more threat, so I would think it would get used by a tankadin, especially if there are multiple Paladins in the group or raid that would provide other useful Auras.
SO... I think that Retribution Aura should be renamed, and that Improved (whateveryoucallit) Aura should also go into the Prot Tree. Blizzard can't call it Thorns like they did in Diablo 2, since Druids already have a buff called Thorns....
Maybe call it Divine Spikes or something silly like that and have Improved Divine Spikes in the Prot Tree. Even though all Prots do spend at least SOME points in the Ret. tree, I don't think many (if any) take Improved Retribution Aura as there are better things to take.
Would putting this into the Prot tree actually help with threat generation? When Protection Paladins get to raid content are they still USING retribution Aura? Would it be worth being able to improve this Aura as a Prot or would the additional threat generation be to minimal to worry about?