The first thing you need to know before you go armory-stalking a popular priest is that high-end players don't necessarily use the same generic spec for every fight when they haven't killed the boss yet. This is especially true if the player's raid is having difficulty with the fight. In such a scenario a priest would put together a talent spec which is designed to give him the best possible chance to beat the fight. As a healer that could sometimes be a spec that allows for the most possible output, but it could also be a spec that provides more utility to the raid, more survivability to the individual priest, or more mana return over the span of the fight so you don't go OOM.
Do top players care about how much healing they do? Of course! But usually not until a boss has been killed a couple of times and the stress involved in healing the fight has passed. When that happens top players definitely start picking up more throughput oriented talents so they can try to set records with their healing numbers; but it's usually just something fun to do until the next content patch is released. Most serious raiders prioritize killing bosses to individual meter performance, and during a progression boss fight they know that an extra 1% or 3% healing doesn't make as big a difference as 1% extra damage does to a DPS.
So how do you differentiate a spec used in progression versus one used during farm content? Easy, just look through the priest's spec and look to see what his talents provide him as a player. If he only has talents that increase his healing output, then he's probably farming all the existing content in the game. If he's got a bunch of seemingly random survival talents that you've never taken before, he's probably working on a specific boss where he finds those talents valuable. Let me go over some possible progression or farm talents.
Evangelism, Archangel, and Atonement This package deal of talents could either be used for a very specific fight (think Halfus Wyrmbreaker) or for fun during farm content. Most of the time if you see a priest using it for raids it's because the content is on farm and there are too many healers in the raid to keep healing interesting. (For progression, as I've said before, there are too many better talent options for healing since Strength of Soul was buffed.)
Inner Sanctum For both discipline and holy priests, Inner Sanctum adds the option to reduce incoming spell damage taken by the priest while using Inner Fire. This is a very beneficial talent since it will help to keep you alive and reduce the amount of mana you have to spend healing yourself. Keep in mind that you miss out on using Inner Will if you try to use this for survival though.
Soul Warding These talents are standard for a disc priest, but a holy priest should consider grabbing them if she's ever feeling short on healing. With Circle of Healing and Prayer of Mending having a cooldown, and Renew only able to heal so much in a given time, adding Power Word: Shield to your repertoire of spells can be quite handy. It also allows you to spam Body and Soul on your allies if you need that kind of utility.
Desperate Prayer This self-heal is an amazing progression talent, and while most holy priests can grab it without making compromises to their spec, a discipline priest has to give something up to get it. If you're prone to injury, or just want to conserve some mana when you heal yourself, this is one of the best progression talents available. Take it whenever you're worried about dying to something that is not entirely avoidable.
Surge of Light For both disc and holy priests, Surge of Light is a fantastic pair of talents ... If you're single-target healing. If you're shield spamming or AoE raid healing, you're going to get limited use from these talents so keep that in mind if you're in a raid where your assignments are more narrow. I think it sounds ideal for healing in 10-mans, but I know many of my 25-man peers skip it.
Rapid Renewal This is one of those talents that some priests might think is standard but actually it's extremely situational. Everyone likes the idea of having Renew on a 1 second GCD for spamming, but when you think about it, how often do you use Renew like that these days? It really depends on your gear and whether you can support upping the frequency that you cast the spell, so remember that when you take this talent.
Lightwell This will be the first talent you drop when a boss goes to farm, but when you're working on it you should definitely keep the spell on hand. Whenever a fight leaves you short on mana or time to heal everyone up during a heavy damage phase, or if your raid is split up between two locations, Lightwell is perfect. And believe it or not, it's much easier to get your raid members to use Lightwell when they can't stay alive any other way.
Blessed Resilience Much in the way that Inner Sanctum is valuable to survival and mana, Blessed Resilience can also help you out if you're in a fight that involves heavy raid damage (like Al'akir.) As long as you're being hit for good chunks of damage, this talent will allow you to heal back up to full faster and more efficiently than you would without it. If you find yourself taking a lot of damage and short on healing in a progression fight, this could be an option.
State of Mind I've come to hate this talent for existing, but I admit there is still potential for it to be useful as a progression talent. Particularly for 10-man raids where a holy priest may be trying to fill multiple roles, being able to switch between different Chakra stances every 24 or 27 seconds could be preferred to the full 30-second cooldown of Chakra. Many boss abilities are on on 30-second or 45-second timers, so being able to switch just a few seconds earlier helps you get ready. So if you find yourself needing to swap between stances frequently in one fight to keep your raid alive, consider this.
Darkness This is one of those talents that could actually be used in either farm or progression content, but most of the time it will be for farm content throughput. Early in tier 11, many holy priests grabbed points in Darkness to supplement the lack of haste we had on our gear. We did this to get an extra tick of Renew. Nowadays though, you can get all that haste through gear and gems, so Darkness isn't all that necessary. If a priest was to take this progression, it would likely be on a fight where he felt short on time to get casts off.
Veiled Shadow This is becoming a standard talent for progression to ensure you don't go OOM. Any serious priest will want to take this during progression, but with the way mana is in Cataclysm it's possible priests will want it during farm content too.
Now that you have an idea when and why a talent is used, I hope that most of you will be able to apply this information to your future raiding specs in the Firelands. If you're running into a problem in a boss fight consider exactly what the root of it is, then look over this list and see if there is a talent you can drop or pick up to make the fight easier. Good luck!
Spiritual Guidance has the inside line on pre-raid gear, valor point and raid gear, and healing strategies for bosses such as Atramedes and Chimaeron. Newcomer to the priest class? Look into Discipline Priest 101 and Holy Priest 101.