Discipline is easily the best on the meters, but is it the best healing spec? The answer to that is based on your current healer make-up, your gear, how well you know the damage rhythm, and if you want to heal before the damage happens or after the damage happens.
I suggest that if you do not know the fights you are going into 100% then you should read Illuminati's Holy guide.
Cooldowns/rotation
Spirit Shell : The single reason why Discipline is so strong right now. This buff causes the majority of your abilities to grant a shield for the 'healed' amount, increased by your mastery/buffs/chance to crit. This should always be used when the raid is about to take a massive amount of damage, or to stop damage so that your other healers can get peoples health up while the shield holds.
Power word: Barrier : Should only be used when asked for on designated locations, or in the event of raid wide damage when people are stacked.
Inner Focus : Should be used on Cooldown to preserve mana, try to combined with Spirit Shell and prayer of healing to get the most use of it.
Archangel : This buff would be maintained when the raid is taking damage. Not during light phases of damage.
Atonement : Atonement healing really sets us aside from other healers, being able to do damage and healing at the same time if one of our greatest assets. However that being said, you should only use it in a few cases.
1. You know that heavy raid damage is going to happen in roughly 30 seconds and you need stacks to use archangel.
2. The raid is taking a very light amount of damage.
3. You are nearing the enrage timer on a boss
4. Random people are taking a moderate amount of damage and is is very unpredictable.
5. People are not focusing on important targets and they need to die Asap (Totems, crawler mines, etc.)
6. You have a temporary Damage buff that will cause the healing from atonement to heal much more then normal.
7. You can hit a target with less then 15% HP to Proc Twist of Fate
You should NOT use atonement if the following is true.
1. You are the main healer and you are carrying other healers.
2. The raid is taking massive damage.
3. Single raid members are taking loads of damage.
4. Someone is standing in the fire.
5. Raidwide Damage will be coming in ~10 seconds.
Prayer of Healing: I'm making a special note here for this spell, it only heals the target that you currently have selected, and the party members that are 30 yards away from that target, not raid members. It does not smart heal, let me say that again IT DOES NOT SMART HEAL. You will need to select different groups to heal those groups.
Combining cooldowns
If possible, before Massive raid-wide damage, use the following abilities to maximize the effect of the shields.
Prayer of healing
Spirit Shell
Inner Focus
Archangel
This should give you a massive increase in Heals-per-second, as well as saving your other healers mana, time, and the effort it would take to heal that damage. If you can try and time your abilities to not overlap with other members abilities, however having a shield never killed anybody ;D
Stats and how they make you heal better!
Spirit > Intellect > Crit > Mastery > Haste
Spirit : Only stack enough spirit so that you are comfortable healing with that amount. I personally dump spirit at 15,000 spirit, but I have healed with as little as 10,000.
Intellect : More chance to crit, heal for more, bigger shields, whats not to love!
Crit : Without a chance to crit your mastery will be less and less and less useful, because as dicipline instead of your healing doing +100% you give a shield for The amount healed and your mastery bonus. This amount also applies to your atonement.
Mastery : Increases Your shield amounts, Spirit Shell numbers, and crit shields. But without a high crit %, doesn't do much on it's own.
Haste : A good, albeit, not great stat for us, with your 4pc you wait to try and Aim for about 10-12% unbuffed haste. That way you can Prayer of Healing 5 times while Spirit Shell is up.
Gemming
Talent build
I heavily suggest using the following talents.
Void Tendrils
Body and Soul
From Darkness Comes Light
Angelic Bulwark
Twist of Fate
Divine Star OR Halo (Depending on fight.)