First of all, my spec is currently 33/28/2, which is level 72. I will also discuss where the last 8 talent points will probably go when we get there. As you can see, right now I'm more Discipline than Holy. Discipline, however, offers many healing enhancements, so it's not as big a hit as one might think.
Before plunging in, let me note that for talents that appear to be PvP-oriented (i.e. are triggered by me getting hit) talents are not up for consideration, and I'll not spend any more time discussing that. Other talents that look good but are not taken for other reasons may be discussed.
I'll hit this one first to get it out of the way since I only have one talent here
Spirit Tap (2/3) - The sole reason to get this is for mana regeneration. 2/3 rather than 3/3 because I'm willing to gamble that I can get by on 66% efficiency on this talent.
This is possibly one of the most misunderstood trees of the bunch, but it truly has something for everyone. My main concern here was to get Reflective Shield and Divine Spirit for party buffs, but there are many delicious treats along the way.
Twin Disciplines (5/5) - Can we be more obvious here? This one cries out "Holy Hybrid, Batman!" all over the place. It enhances damage output for grinding, and healz for raiding ((Omigawd! I just realized that this only affects instant spells. I am now seriously reconsidering Unbreakable Will (5/5)! I can think of more than a few places where that comes in handy.)) .
Silent Resolve (3/3) - This is another one that has Hybrid written all over it. Healer aggro is so intense sometimes, especially with adds. If you're a healer, you need this to reduce the chance that Mr. Ugly's friends want to dance with you.
Improved Power Word: Fortitude (2/2) - If you're one to like PW:F, the improved version will make you happy, and this also applies to the party-wide Prayer, which will make your tank happy. I adore happy tanks.
Improved Power Word: Shield (3/3) - This one speaks directly to survivability when grinding, obviously. Its absence is very noticable if you don't take it.
Meditation (3/3) - This is, again, a no-brainer for a hybrid spec. You need mana for healz, and this improves the regeneration during combat by a healthy amount.
There's actually not much for me to like about this tier.
Mental Agility (4/5) - I took this simply because I had to take something from this tier, and the others were not as compelling. This one is very situational, only affecting insta-cast spells, but there is good news there - Renewal is an insta-cast, and something a healer is likely to use often.
Divine Spirit (1/1) - If you're healz, then you want Spirit. This one's a given.
Improved Divine Spirit (2/2) - ... and increase spell damage, you say? Mages and Warlocks ask for it by name!
Mental Strength (2/5) - My only regret is that I don't have 3 more points for this one, because it makes grinding go by so much faster.
Focused Power (2/2) - This one increases DPS and healing, therefore is pretty obvious for a hybrid spec if you're this far into the tree.
Enlightenment (3/5) - This is another one where I wish I had two more points just floating around. Stamina, Spirit, and Spell Haste in equal measure.
Reflective Shield (3/3) - This one is a treat for grinding, and if you need survivability this one will become beloved to you. It does not improve the shield's durability, but almost half the damage that it takes will bounce back in the face of the enemy.
This thing is golden against attack totems, imps, and other small units with high DPS and low health - they usually kill themselves when attacking you!
Against the bigger units, it's an additional source of damage, and it crits using your crit table, or so it appears.
So, with 8 points left to spend, how would I dump them in this tree? Well, I can't move past Tier 7 without either spending points in the rest of tier 7 - which is unlikely (PvP *cough*)
The most likely would be to take the last two points of Enlightenment and then skip on over to the Tier 8 talent, Rapture, which is another mana-replenishment talent that just happens to mesh well with Holy. This would push me further towards sustainability rather than Massive Healz FTW. There would be one point left over, and there's nothing in Tier 9 that interests me so I'd toss it to the Holy tree.
The first unusual thing you will notice in this tree is that there are quite a few damage-enhancing talents. Everyone thinks "Healz" when they see "Holy" but the original conception of the Holy priest was not exactly "Healbot" - it just ended up that way.
Improved Renew (3/3) - I think we can agree that increased healing effect is good, yes?
Holy Specialization (5/5) - Note that increased chance for Crits applies to Holy spells - both DPS and healz.
Divine Fury (5/5) - Again, all Holy spells get the benefit, not just Smite but also Greater Heal, etc.
Not my favorite tier.
Desperate Prayer (1/1) - Unlike most PvP-ish damage mitigation talents, this one is wholly effective provided you get it off in time. Roughly 60% self-heal, insta-cast. This is a major life-saver. Take it and find a (very prominent) home for it on your action bars.
Blessed Recovery (1/3) - There is only one reason to take this talent, and that is to get you to the required number to move on to Tier 4. Another situational, PvP-ish talent, I would not be at all interested in it if it weren't for the even lesser attractiveness of other talents not taken. This one at least has the possibility of helping save the day in the direst of straits. But don't count on it, even at 3/3 ((Hey, since I'm probably respeccing for Unbreakable Will anyway, maybe I'll find something I like better.)).
Holy Reach (2/2) - Because extended range for your healz is a Good Thing.
Improved Healing (3/3) - You spend a lot of time healing in an instance. Anything that reduces the mana consumption of that activity will extend the amount of time you can keep actively on-station healing. Win.
Searing Light (2/2) - OK, I confess, this one does nothing for healz. It's all about the damage. If you need more healz, you might consider dropping this one in favor of one of the upcoming tiers. On the other hand, it does make grinding so much easier.
Healing Prayers (2/2) - This one is very limited in scope, and normally I wouldn't take it because of that, but note that it is a whopping 20% saving on mana for one very big mana-eater spell, and it starts to look more palatable. Highly subjective.
Spirit of Redemption (1/1) - This one is an absolute must-have for a healing priest. It quite often makes the difference between victory and defeat. You get a few seconds to get a last HUGE heal off (without a fear of reprisal) and maybe set up a hand-off to the backup healer, if you have one. Also, when you take this, come up with a good exit strategy, set it up, practice it, and get it down so that you get maximum use of this talent.
Surge of Light (2/2) - This one is hard to quantify, and may be totally not worth the two points. What it does is akin to Nightfall in the Warlock talents. It works for healz, too. With a 50% proc chance on each crit, it probably will proc a lot during healing, and insta-healz are a good thing when you can get them. I say you will probably have to evaluate it, as I am. Thus far I have been pleased with it.
Spiritual Healing (1/5) - Obviously I am working on getting this one tanked up. Fully talented, it will be a 10% improvement on healz. All healz.
Beyond Tier 6
The other eight talent points have many possible homes here. I could go with 5 in Spiritual Guidance for enhancements in both healz and DPS. Stacking Spirit will help here.
Holy Concentration looks very nice, and I'd have to take two additional points in Spiritual Healing, which is not a bad thing. The remaining 3 points can be spent topping off Spiritual Healing and contributing one final point to either Serendipity, Empowered Healing, or Lightwell. It's largely a tossup, though I would not take the latter unless the party members knew how to properly use it. I could also toss those last 3 points into the Discipline tree.
My inclination is probably to take the Lightwell simply so that I have the option in raid situations. Something where I can gather up with a bunch of DPS that will be taking damage over time, such as with the Nightbane fight. But it's still up in the air.