I don't want to get too much on Blizzard's back for their "bring the player, not the class" mantra. It has been quite successful in a number of ways. People who don't think it is successful need to take a look at raid composition at the end of BC, where you needed a shaman in every group and multiple shadow priests just to get things going, and where rogues did the most damage but needed so many other classes to enable them to do the most damage that there often simply wasn't space for them. It was a real mess.
Raid buffs are still a very tricky situation in 10-player raids. You want to try to make sure you have them, but having them all is pretty impossible. Blizzard has indicated that in 10-player raids they expect you will have some but not all raid buffs. And that's fine, of course. Raid buffs tend to give a little more than a 2% bonus to overall raid damage, and most of them apply multiplicatively. So they assume you have all 17 raid buffs for 25-player and some less number for 10-player when they set the health of the bosses.
Of course all raid buffs are not made equal, so I took one of my raids and did a little analysis. First, I know the percentage of the total damage each person did, so from there I figured out what portion of each person's damage was physical and what was magic (as well as what part was a "magic strike" which deals magic damage but hits and crits as a swing not a spell). I figured out what portion of each person's damage could benefit from haste rating and an approximate value for how much benefit to damage each person gets from crit. All of this allowed me to compile the following table:
While some buffs are barely a 1% bonus and others manage to get over 3%, the 13% spell damage buff is well over 8% bonus to overall raid damage. This is not a caster stacked raid either. The raid in question had 9 different dpsers swap in and out through the night. The number 1, 2 and 3 damage spots were a rogue, a hunter, and a shadow priest. But almost 17% of the rogue's damage is affected by +13% magic damage, it would be more for mutilate spec, I believe. Almost 50% of the hunter's damage and 40% of the death knight's damage was magic. The paladin was half and half as well. Basically 13% spell damage gives 13% damage to half your raid and half that to the other half, so it's more than twice as good as the next best buff.
And then there is the way it is handed out. Balance druids put it up on the main target you are attacking only. Any warlock can put it up, but they use a GCD to do so for each target and have to take a personal damage loss by not using other curses. Unholy death knights put it up on all enemies you are facing by doing their normal damage rotation. So if there are multiple adds being burned down, only one spec of one class provides a buff that is over twice as good as any other buff.
One of the most mind-boggling things about all of this is that there used to be a 10% spell crit debuff that mages put on the enemies and Blizzard toned it down to 5% because they thought it was too powerful. The 5% magic crit debuff on the enemy is worth around 1.75% damage, so doubling that would bring it to 3.5%, which is better than the next best buff, but still less than half of +13% spell damage. If instead of halving it they had doubled it, it would still not be as much of a problem as 13% spell damage. What's more, while it is limited to a number of classes and specs, all those classes and specs apply it in a relatively equal manner by putting it on targets only with single target spells.
They argued that because crit gives lots of other benefits, it was more important to drop the crit debuff than the bonus damage debuff, but in reality 1% spell crit is worth less raid damage than 1% magic damage. This is in large part because the 1% magic damage affects many things (like rogue poisons and explosive shot) that 1% spell crit does not. It is also because if things are only critting for double and you have 40% crit already, then 1% crit only increases your damage by .7%. Things crit for more than double in many cases, but also people have a lot more than 40% crit in many cases. Plus, not everything can crit, but everything does damage. Crits may proc buffs or other effects, but most of those work either as just bonus damage (ignite) which effectively increase your crit multiplier, or they have high uptime with existing crit values and upping crit by 5% might change uptime from 90% to 91%.
But 13% spell damage isn't even the biggest problem on the buff/debuff scene. I used an Anub'arak log to estimate the percentage damage value bonus of mortal strike on that fight for phase three.
The mortal strike debuff can be applied by a pretty wide range of classes and specs, since you can get it from any hunter, any rogue and two specs of warriors. But in the hard phase of the toughest encounter from the last tier of content, in order to make up for the healing it removed from the boss you would have had to do 20% more damage. Assuming you couldn't do 20% more damage, that would not mean the fight would go on a little longer. It would mean you'd have to kill another wave of adds that you would otherwise have just ignored while you finished him off. Of course in that time you are killing adds he is healing, which will give him time to summon another wave of adds. In fact, when we were fighting Anub in mostly 226 and 232 gear and we forgot to bring an MS effect to a fight we literally weren't doing any damage to him. The amount of damage we did to him between add summons was completely healed back by the time the adds were dead. We could not win without this debuff, even if he had no enrage timer we just weren't making progress.
Last night we were working on Lich King and talking about what we would do if we didn't have paladins to hammer of justice the Val'kyrs. Availability of long stuns is not something that I normally try to plan raids around, but this section of the fight would be hugely more difficult without paladins.
And I posted just recently about paladins and Valithria Dreamwalker. The challenge that fight presents without a holy paladin is astronomical compared to how hard it is with one. While I can forgive this one as a one time thing - especially since Beacon of Light is a problem in so many different ways - it is another example where the difficulty of a fight hinges hugely upon raid composition.1
Because raid buffs are still very powerful, Blizzard's "Bring the player, not the class" is only half honest. They do want you to think about raid buffs and bring a mix of people. It is not their intention that any raid will be good enough. You still need tanks, healers and damage dealers, after all. Runescrolls and drums are making previously necessary classes even more skipable.
But when a single debuff makes the difference between winning and losing - not because you had a 1% wipe - because the fight is just designed so that you can't win without it, things have swung back way to far into "Bring the class no matter who is playing it" territory. We would have a better chance against Anub with a hunter who went afk with one of those little birds tapping his aimed shot button than we would with a serious high dps class in that spot if we had no other source of healing reduction. Enemy heals are absolutely huge (they have to be to make them worth using) so that debuff is overpowered any time it is at all useful. We should not be so dependent on a single ability.
I think that healing reduction debuffs should just go away completely. Find a different way to make classes competitive in PvP and reduce healing on PvE enemies to account for the fact that it is gone. Or if the PvP problem can't be solved then just make bosses who heal immune to it. At the very least make the 10-player version immune. As lame as that is, requiring that debuff is even more lame.
1. Fun Fact: A group of 10 ilvl 245 geared holy paladins would beat the Dreamwalker encounter before the first blazing skeleton.