At this stage in the game I'm fairly certain that the community as a whole is made up of a bunch of whiners - and specifically I'm talking about Paladins here - and more specifically I'm talking about Mastery.
Back in the days of Wrath I lusted after one single item - Val'anyr. Forget the fact that it was legendary, had an awesome animation, or helped me get over my priest bubble envy - this bad boy had the most amazing proc for healers; Stacking shields. This was huge, and close to being as overpowered as you could get, without being game changing. 15% of all healing (while the buff was up) was passed on as a stacking absorb shield. Mmm - I loved it.
Then came Cataclsym, the nerfing to a 10k max shield, and the inability to put Heartsong on it. /sadface.
Mastery was supposed to be a quasi answer, but it didn't stack, didn't last for very long, and was generally accepted as being extremely lackluster. We've seen a few small buffs to this stat since launch; increased percentages, increased time, and now - stacking shields.
Well color me happy
Shitfire and potatto sauce - we've finally gotten it. Sure there's no awesome animation to go with it, but we finally got stacking shields - that last for 15 seconds. This shield is applied by the majority of our heals, and it stacks! We have essentially been given the power of a legendary, without having to do anything other than equip the gear we normally would anyway. Plus, because it is tied to a stat, you now have the option to decided just how fast you want those shields applied. The world of mathcrafting just got a new contestant in my opinion.
Balancing your stats
So as it stands, any Holy-Paladin who reaches whatever level is required to get the Mastery skill, automatically get's a 10% shield applied when they heal. Ten freaking percent, and all you have to do is show up. So now it's time to start looking at stats, and seeing where we can land and where I think we'll be with some ideal settings.
First - Spirit. No mana = sad panda, so stack this until you're comfortable where you're at. Personally, I can never have enough mana, so I stack Spirit like it was the last candy available.
Second - we want to make sure we keep conviction up at all times. That's way too much healing to lose simply becuase we let a buff fall off. With heals critting for 100% in 4.2 as well, extra crit won't be as much of a waste, but we still don't need 'that' much. Anything in the 13 to 18 percent range is probably fine.
Third - Haste. This is the big one as far as I'm concerned. Right now I'm certain that it's impossible (at least not without BLust and everything else) to reach the 1 sec GCD for HL/DL. So, what we really want to look at is Holy Radience. 6% haste gives us an extra tick of HR, and we don't pick up another until 14.5%. This is reachable (by reforging) in the current tier without too much effort. Pushing for 23% is probably not possible, at least at these gear levels (or giving up a lot of stats).
So where does that leave us?
While an extra tick of HR is nice, that's really not going to provide 'that' much extra healing. So you need to find that sweet spot. Personally as long as I'm under a 2sec cast for DL/HL, I consider myself to be ok (until I can hit 1 sec, I'm not going to worry about it). This means I can play with more mastery without putting myself in harms way. Kurn did a chardev where she maxed out her Mastery and sacrificed her haste, and came up with 8% haste and 19 mastery. This is a flip from her 15% haste and 9 mastery in her current raid gear.
So let's look at those numbers. 15% haste will provide you with that coveted extra tick of HR, while 6% will still give you 2 sec HL casts, and a lot of extra Mastery. If you're raiding 25s, with a lot of grouped fights that HR will really affect, this may be worthwhile. In 10s however, you're not always going to be getting the most from your HR, so running one tick less won't make/break your game.
The big downer
Now the Mastery changes aren't all roses. The drawbacks still exist, primarially that your BoL target and PoTI doen't casue your shields to stack. HR doesn't affect it either, but that's ok. What we're really concerned about is shields on the tank. Here's the deal though. Can you imagine if all the healing your tank recieved via BoL was affected? You'd have huge shields, and tanks that were nigh-on indestructible. When you add in the 100% HL transfer, and 200% crits, it would really get out of hand in a hurry. By the time 4.3 rolled around Mastery wouldn't transfer anything, and we'd be using bandages.
The wrap up
Face it - you got a free Val'anyr, just for rolling a Healadin. Never add another drop of Mastery, and you've still got 10% shields affecting the majority of your targets. Stack Mastery now, and you can get about 30% of your heals transferred as shields, for something you were going to do anyway. That my friends, ain't too shabby.