I spec'd holy up till around 30. Then I spec'd prot. I twinked @ 49 for about a month, got tired of killing horde in the BG day in and day out.
I'm now 53, walk into WPL, and go into the graveyard just off of the Flightpath, and I tested myself.
10 50-52 mobs at a time, roughly 1800-2500 xp gained in about a minute.
Its really simple, all you have to do is find the right mix of mobs to grind, and just beat the living tar out of them, well, let them beat the living tar out of themselves, whatever the case may be.
The key is as everyone else has said, more hits = more procs, more procs = more dmg or dmg mitigation in relation to redoubt.
Here's my armory and obviously I realize my gear is far and above more than a lot of guys will have at my lvl, but seriously, you can get close and still be taking on countless #'s of mobs.
I will generally judge wisdom, and seal it too, more so than life, I find myself burning mana constantly, as I'm a ***** for consecrate, I wanna burn em down as fast as I can, and that 36-40 dps on every single mob, is a huge buffer when it comes to taking massive groups down.
So I burn mana way before I burn HP. If I didn't use consecrate, I could probably go all day and night without a break in the action and never once have to bandage / bubble.