Each paladin spec has it's own mechanic to regenerate mana. Holy pops Divine Plea once a minute to get 25% of total mana back. Protection has Guarded by the Light to get 100% of total mana back per minute. Retribution has Judgements of the Wise which returns roughly 190% of base mana back per minute.
The interesting thing is that the amount of mana returned is not that far apart. If we say Holy has a total mana pool about 5 times base mana, then Holy gets 125% base mana back per minute. Ret is a bit of an outlier, but not that much higher.
Rather than having three separate mechanics, maybe it would be better to have one mechanic that all three specs use. I think the best mechanic out of these three is Retribution's JotW. Judgments are an iconic part of the paladin class. Plus, Judging is fun. A giant golden hammer drops out of the air and smacks your enemy in the head. Never gets old. Hitting Divine Plea once a minute is not particularly fun. As well, most tankadins seem less than enamored with the way Guarded by the Light works.
Suppose JotW's functionality was built into the base Judgement spell, so that each Judgement returned a net of 15% base mana. JotW could increase the mana returned by another 5% or 10%, to keep Ret the same.
That way, each spec would be encouraged to Judge a lot, and Judgement would be the primary method of regaining mana. We could then remove Divine Plea from the game.