The only major differences I find doing 10 vs 25 (I raid in a 25-man but we sometimes clear 10man for various reasons) is coordinating things like SS. If you have two discs, decide who SS's which groups in order to not "override" the other discs bubble, causing wasted shielding. If you're the only one, try to utilize your SS in a way that groups you know are likely to take most dmg are certain to have full SS on them (unlike on 10man where you can have full SS stacked on the whole raid).
My guild tends to run with only me as disc (except horridon where we have 2) but if you use more you might want to decide on things like who gains Rapture from which tank (more than often you use 2 tanks in 25) as it gets to be a pain in the ass if 1 disc shields both tanks all the time disabling your Rapture "on demand".
I find Cascade a lot more useful in 25man, I rarely ever use it in 10 man. DS is still strong for some bosses though, such as megaera and basically any where the raid is stacked enough for long enough periods.
Due to the increased amount of healers in 25 I find myself using more atonement in 25's as the smartheals, whereas on 10man I find it easier to pre PWS as the dmg income is a bit more coordinated in a way, as there are less people to keep track off. In 25 random people take random dmg more often, making atonement nifty.
Not really sure what else Id find that different, guess we have more mana cooldowns (think mana tide and hymns) on 25 allowing more random "big heals" / shields where needed, as you're likely to get more mana back after those "oh shit" moments.
Those are prolly the major things I find different, but all in all it's pretty much the same. I keep the same talents part from swapping out cascade during most bosses in 10mans.
I honestly feel like I'm using slightly less atonement in 25s, slightly less shields and more PoH's/binding heals to deal with the larger amount of raid damage, there's generally always be a group where PoH will heal 4-5 targets and because you have a bit more mana to spend (with all the regen tools in 25+I play with a bit more spirit). Also worth noting that your spirit shell usage differs a lot, for 10 man you can generally not cast PoH's during the entire shell since it'll cap out, in 25 man you want to do your best to fit in as many as possible, possibly even by popping a PW:S and getting borrowed time before using it.
If you are using 2+ disc priests you obviously have to coordinate about rapture, but that applies for 10 man as well. Same with PoM getting a bit weaker with many priests.
I tend to agree with you, Cookie. Atonement is great for random damage in 25's, but is not such a straight-up no-brainer as it is in 10-man, where, percentage-wise it works as a relatively effective AoE healing tool. For high damage AoE phases where you don't happen to have SS up, you will PoH and/or BH (glyphed hopefully), along with copious use of Archangel and try to use PW:S mainly to get the Borrowed Time buff for your next cast. Also, Inner Focus is a very good tool here (more so than 10-man), but unless you're tank healing, it will be on a 1-min CD.
"Shit Happens" - Well, imagine if a pegasus had the bowel-control ability of a pigeon, there'd be a lot of cases saying the following: "Cause of Death: Killed by a giant flying piece of shit...."