Spine of Deathwing.
This fight sucks for the damage dealing role. There's no reason to sugar coat it; I'll be blunt and up front about it.
The first thing you'll notice is the lack of meaningful environment graphics. It takes place entirely on Deathwing's back with nothing but indistinct, swirly clouds as the surrounding (technically you can see the ocean below). We pass no landmarks or creatures along the way to the Maelstrom. The animation is robotic - his wings flap and head bob, but with the entire torso remaining static there is no sense of motion. It feels like we are on a stationary platform that has these odd appendages and apparently we're in a wind tunnel because swirly things go by? But every once in awhile the wind tunnel rotates to simulate a barrel roll?
All of the graphic and animation pet peeves are secondary concerns for me, though, at least as far as this post in concerned. Don't get me wrong, having an interesting environment is crucial in my memories of all of my favorite fights. But when we are talking about an encounter that we will attempt not just two or three times like some flashy five man, but 100+ times at a minimum, the graphics can enhance or detract from the experience but the core is the actual fight mechanics - the actual gameplay. I won't try to comment on how the fight feels for healers or tanks, but for DPS I think it fails considerably on this aspect.
Here's a quick rehash of how the fight works: You land on his back and there are four corruptions. Each time you kill a corruption a single Hideous Amalgamation will spawn from that hole and Corrupted Blood will also periodically spawn from that hole. A common strategy is to kill the first three Corruptions, force a barrel roll to throw those Hideous Amalgamations off, and then kill the fourth Corruption. Another Hideous Amalgamation will spawn as well as another Corruption somewhere (you can't get rid of all Corruptions - if you kill the last one, a new one will always reappear). When Corrupted Bloods are killed they form a pool over which the Hideous Amalgamation can be brought to soak them up and gain one stack. If the Hideous Amalgamation is killed when he has nine of these, he blows up and can knock a plate loose - which allows you to target and attack Burning Tendons. When killed, that plate flies off and it's basically a repeat of the last phase with more Corruptions and a slightly bigger area on which to move.
This is all the same as normal, except a few things are stressed a bit more. Burning Tendons are stressed a lot more. They have more health and less time attackable so it takes two times blowing up a Hideous Amalgamation to kill them. Six amalgamations, three Burning Tendons in total. This prolongs the fight which stresses healer mana, means the raid will have their max health reduced by up to 30% before the end, and means more Corrupted Blood will spawn until its just an absurd amount by the end of the fight. Almost all of this is a matter of control. The fiery grips from the Corruptions require pre-casted attacks to limit their raid damage. Corrupted Blood has to be killed intelligently. Killing Hideous Amalgamations more efficiently will end the fight faster so DPS is important here, but it's still mostly about control and just playing well overall. There is no magic time threshold in which you need to kill the Amalgamation. If it takes 10 seconds longer to get one down, it's probably not ideal, but it's probably not going to matter much either.
What does matter is that 18 second window during which you have to get that tendon down to 50% the first time and finish it off the second. It is incredibly important because if you have to do it three times you will end up with so much blood by the end of the fight that your tanks will be overwhelmed, and everyone loses 6% more health due to degradation. It has to die in two. With the rest of the fight being mostly about control, this means that 18 second window is pretty much the only important aspect of DPS on this fight and it is stressed for every single DPS in the raid equally.
Classes are not at all balanced for a theoretical 18 second window. Nor should they be; it is way too specific to one particular encounter and would require classes to be too similar overall. That means although class stacking is incredibly important here (less after the 5% nerf, but even still) it is a failure of the encounter design, not the class design. My hunter doesn't need the burst potential of an arcane mage going forward; I have other strengths that can hopefully be valuable. What my hunter needs is to not be expected to fill the exact same role that some other class would in a highly specialized scenario. That's really what the problem is; it's not that an 18 second burst is important, it's that it's important to every DPS and nothing else is nearly as important.
I think they could have done one or two things here and still made a short burst window important and interesting. First, make it so that it's not the only DPS requirement going on at the time (fiery grips still happen, but this is a minimal concern honestly) and so that you wouldn't want to have ALL of your best DPS on it. Maybe you want to have five on it, while the rest of the DPS are dealing with important concerns which they cannot afford to neglect to assist the Tendons team. So you take your five best players/classes for that role and assign them to it. Or maybe if you are really good and experienced at it you can drop down to only your four best people for that role, while the fifth can assist in other roles. The classes that simply aren't good at that role don't have to be; their expertise is useful on other aspects of the fight. This approach requires several moving parts. I think Lady Vashj phase 2 was a great example of doing this well. There were damage requirements but they were a bit more seamless in that the goal was to get your assigned targets down before the next one of that type came up, but you could afford to have a little bit of overlap if you handled it well. You had one or two people that could kite well in charge of the striders. You had people who were mobile and quick able to handle the elemental spawns. You had people who weren't that good at either but could take down the elites before that tank was overwhelmed. And you had all of those types working together for the tainted cores. You didn't have to be really good at one role to be beneficial, you had to be decent at one of three or four roles, and even then there was still the phase 3 concern. What a great encounter!
Even if you don't have all of those moving parts at the same time, you could still stress other strengths at different points in time. As I said, dealing with fiery grip and overall Hideous Amalgamation are important but they aren't really stressed in regards to damage. They are more about control and just about everyone can handle them well enough. If you had Hideous Amalgamations disintegrate after X seconds and this is really hard to reach, suddenly that 18s burst isn't the only thing anyone cares about anymore. Maybe there's a second phase where aoe is really important and a raid full of arcane mages wouldn't be able to handle it. Or some people have to hop on a floaty disc Kalecgos throws at you and go take care of something on the wing. There are a lot of possibilities.
Lumping all damage dealers into a single, amorphous role with the same duty (and only a single, highly stressed duty) but different strengths and weaknesses is poor design; and it is poor encounter design not poor class/spec design. I think we actually see this all over the place in Dragon Soul, it just happens to be stressed in the most obvious way possible on Spine of Deathwing.