This is bad system, but it's exactly what paladins use.
Holy has no passive regen. Zero. Nothing. But so what, Illumination gives the equivalent of... let's see... Casting max rank Holy Light at 840 mana/cast, with Light's Grace it will be 5 casts per ten seconds, or 2.5 casts/5. With a 40% crit chance that would mean an average of one crit every 5 seconds. That gives an average of 504 mana/5. That's pretty nice, isn't it?
Unfortunately spamming max rank holy light is also a great way to drain mana. Even with Illumination it will drain 1596 mana/5. That will wipe out a healthy 16k mana pool (does anyone even have that much mana?) in less than a minute. To compensate for this rapid drain paladins can use lower ranks or switch to flash of light. This will cost less, but also give less regen and less healing per second. Still, mana isn't a huge problem for paladins in PvE since they do have mana/5 on gear and can use potions.
But in PvP this means that an OOM holy paladin is totally fucked. PvP gear has zero regen and the tree has no passive regen. It isn't the healing that removes mana, it's the viper sting and mana burn. Cleansing or LoSing those means losing healing output.
Ret isn't any better. The only regen in the tree is a from judging seals, and it only gives 80% back. Sure ret isn't a mana hog, but anyone can go OOM with no regen. Belfs are a little lucky with seal of blood, but at 1000 white DPS (1000 x 35% seal x 10% reflect x 10% SA) the healing still only gives 17.5 mana/5. Judging gives some more, but we're still looking at tiny regen.
Prot isn't in great shape either. In proper gear a boss should do just about enough damage to keep up a solid aggro rotation. But what if the boss isn't hitting you hard enough? You're fucked. I won't pretend that warriors have it easy either, but they can at least pro-actively generate rage through white attacks while paladins have to take damage, we have no other source of regen unless we're going to break out SoW. Our taunt costs mana, so losing a mob at low mana can mean being unable to taunt, or if you can, you're too low to generate another bit of aggro to keep the mob.
I don't want to get crazy here, but it would be nice to have some regen in out trees or on our gear, without sacrificing needed DPS stats. Shamans are full of great talents for this class: int-> mana/5, regen from white attacks, and unrelated to mana, AP->SP. I realize they're a little hippyish, but does that give them a monopoly on Green? Grass.