It's not in the patch notes, but the data-miners for Warlords of Draenor have discovered a plan to give all healers a form of active mana regen. For Holy Paladins, this means Divine Plea becomes a Holy Power finisher which returns mana:
3 Holy Power
Instantly regain 4.5% of maximum mana.
Now I don't know if this is final, as it wasn't in the latest patch notes. But in my opinion, active mana regen is a bad idea for healers.
First, it is an extra button. Right now, healers may have 5-6 dedicated heal buttons, and we'll have to allocate another button for mana regen. When Blizzard is trying to cut down on ability bloat, adding more buttons is not a good idea.
Second, the presence of active mana regen will invariably make fights more deadly, leading to a less fun healing environment.
Essentially, from a healing perspective, a raid fight is divided into periods of heavy damage and light damage. During heavy damage, mana consumption goes up. During light damage, mana consumption is minimal. A properly designed fight ensures that the healer uses all their mana.
As a heavily simplified example, let's say a healer has 1000 mana. During heavy damage, she spends about 10 mana/second. During light damage, she spends about 5 mana/second. So a perfect fight might require 60 seconds of heavy damage and 80 seconds of light damage.
Now add in active mana regen. Active mana regen means that during the light damage periods, the healer spends even less mana, and may even have positive mana gain. Let's say the healer moves to 0 mana/second because she's hitting her active mana button. That means that, in order to challenge the healer, the fight must now have 100 seconds of heavy damage and 40 seconds of light damage. Or alternatively, heavy damage must be more extreme, in order to make the healer spend more mana.
Both paths lead to deadlier fights, fights where mistakes are more likely to result in a death.
The history of WoW healing has been that whenever mana regen increases, throughput increases, and damage skyrockets to balance, and the healing environment becomes less forgiving, more spammy, and less fun. Adding active mana regen is just to trigger that cycle faster, and put another skill barrier between those who are the best at squeezing in regen abilities, and those who are not as good.
In my opinion, the best healing environments have resulted whenever "overheal" becomes important. When healers are focused on reducing their overheal and becoming more efficient, incoming damage is at a reasonable level and fights are simply more fun because people aren't getting destroyed in two or three global cooldowns. Whenever efficiency gets thrown out the window in favor of throughput, healing becomes less strategic and more frantic, and just less fun.