Dinaer at Forever a Noob had an interesting point today about the 10s vs 25s raid difficulty balance for Cataclysm: healer-to-tank ratio and its impact when tank health is made equal. While my initial reaction was "25s don't let all 5-7 healers spam on one tank! Or so I thought?!"... I read back through his post again and did some quick ratio calculations and saw that he did still have a point.
Here're my thoughts on the matter:
The relative tank/healer/total-raid-size ratio is different for each raid size. Currently, 10s run either a 1/1 per party or 1/1.5 per party ratio (2-3 healers); 25-mans run between a 0.4/1 per party to 0.12/1.4 per party ratio (2-3 tanks, 5-7 healers). Simply put, there are more available healers than tanks per party in a 25-man raid, allowing for more focused tank-healing without that tank's healer having to worry about raid-healing at the same time. To give an example, 2 healers in a 10-man with two tanks each have a tank to heal AND the rest of the raid, while in a 25 they will have the raid-healing job taken over by 3-4 other players, in addition to some extra hots, bubbles, splash-heals, cooldowns, etc. to help with the tank healing from the raid-healers.
Why does this matter? Well, when gear and raid buffs are homogenized in Cataclysm, the tanks should, in most cases, have the same health pool regardless of their raid size. Damage from the boss will have to be balanced with consideration towards how many healers (in terms of HpS) will have their sole attention on the tank... and to some extent, how many raw healers there are in the raid who have tank-saving cooldowns available to help out in emergencies. Otherwise, if it's the same damage to match the same tank stats, then having more healers available to help that tank in the 25-man (or even just having the complete focus of one healer who doesn't have to OT or raid heal as well) will make it seem easier than in the 10-man. Similarly, designing the boss to do more damage in the 25 (to counteract the higher concentration of healers and cooldowns) will make the tank seem squishier than in the 10, and run a higher risk of a tank being one-shot in the 25 when they could've taken the hit in a 10.
There are other ways to counter this, however: the 25 could require more concurrent tanks (be it by adds or damage-sharing links like Lana'thel/Marrowgar), or adds may do more damage in a 25 than in a ten (requiring more attention from the 25-healer and letting the ten-healer do raid heals in conjunction with healing the add tank), or raid damage could be made more avoidable in the ten, freeing up the "raid healing" to allow those healers to focus on their tanks while keeping the raid healers busy in the 25.
It may also be that 3 healers becomes the accepted standard for raid-balance purposes on 10-man fights (as it has leaned more heavily towards in ICC with 10-strict guilds), allowing at least one healer as a "raid healer" who can cross-heal while allowing each tank a more focused healer of their own, and mechanics in 25s could be made to consume the higher number of available cooldowns to balance them out.
It's stuff like this that makes me happy I'm not the developer/designer tweaking these mechanics :)
~~Please read the comments for further discussion on the topic, as many great points have been raised! ~~