What you fight matters more than you think
For better or worse, specs generally have certain strengths and weaknesses attached to them. This allows for the gameplay to be more dynamic, for every spec to feel more unique, yet this inevitably sets the game up for failure. To achieve the perfect balance that players often say that they want, no spec can be unique in any way; no spec can have any strengths nor weaknesses unless all specs have those same attributes. This is the phenomena we experience which creates spec niches.
For example, currently a fire mage will have higher AoE than an arcane mage, but an arcane mage will have higher single-target burst than a fire mage. On the surface that might seem rather innocuous, but it's a strong determinant over which is considered by the community as "the spec" that a player should use. Spine of Deathwing is a fight that hinges on nothing but the burst damage that a player can do in a short time frame, which is arcane's strength, therefore arcane performs far better than fire on that encounter. Conversely, Yor'sajh has extremely high AoE requirements, which favors fire instead.
How specs perform on Patchwerk style encounters does go a long way in selecting what the "spec of the month" is, but it isn't the only factor. Tier 11 was a fantastic example of this. Balance druids didn't, and still don't, have strong Patchwerk DPS in comparison to others, they run more middle of the pack, yet they dominated that Tier. While their far too strong set bonus played a part in that, a larger factor was that the encounter styles of Tier 11 played to their strengths.
Balance druids were great at multi-target and AoE damage, which comprised a significant portion of the encounters players faced. Halfus, Maloriak, potentially Magmaw, Twin Dragons, Ascendant Council, and Cho'gall all features AoE or dual-targets, and in all of them, balance druids shined. By having more encounter mechanics favoring their strengths, balance druids became an essential part to many raids.
Raid and spec favoritism
Specs have always fallen into a specific niche list, especially for pure DPS. That's the AoE spec, that's the single-target spec, that one is for movement. We see it all the time, and it those concepts which truly dictate how a spec is going to perform overall within a given raiding tier. Blizzard generally tries to find a balance with their encounter design by providing a mixture of all the different styles of encounters that you can face, but they still often fall into a pattern with every tier.
ICC had a lot of heavy movement encounters, and movement DPS was the hot topic of the time back then, players, especially ranged characters, were very concerned about their movement DPS going into Cataclysm; to the extent that it was a directly stated goal by Blizzard to equalize movement DPS. Tier 11 had an strong emphasis on multi-target and AoE DPS, which became the hot button issue then. Firelands had the most focus on control as well as AoE DPS, and now Dragon Soul is mostly about pure single target damage.
Raids come in flavors, and it's these flavors that heavily control which specs the community at large considers to be viable.
Changing the value of abilities
Damage is not the only thing which encounter style dictates either. Mechanics also control how useful player abilities are as well. Here's a fun little tip, did you know that the associated cost for arrows was actually justified in Blizzard's original design as a means of making up for the reduced repair costs that hunters incurred from raiding? It's true! Due to Feign Death's ability to allow a hunter to skip dying on a wipe, Blizzard fully believed that hunters suffered lower repair costs than the rest of the raid and thus charged them for arrows. This was mentioned by GC back in WotLK when the viability of Feign Death was brought up in the hunter community.
It's more than just silly abilities such as that which encounter mechanics control, though. Again, I'll reference balance druids. When Typhoon was first added to the balance druid toolkit in WotLK, there was no PvE use for the spell. It wasn't really used for damage, there wasn't really a need to knock mobs back outside of soloing, so whether you had Typhoon for raiding or not was entirely up to the player. Then Blizzard made Saurfang and Typhoon became an mandatory ability for controlling the adds.
Ever since then, Typhoon has become a raiding staple, but only when the encounters demand. Within Dragon Soul, a balance druid could give up Typhoon and you'd never no the difference, but it was horribly important during T11 and T12. Look at Firelands. Beth'tilac, Rhyolith, and Ragnaros all benefited from Typhoon. Three out of seven encounters is pretty good for a niche ability that only two specs have.
Yes, this can apply to damage abilities too. Rogues have a hard time using Killing Spree on Ragnaros, feral druids aren't able to Shred Ultraxion, some adds are susceptible to being stunned, which changes how Deep Freeze works for frost. Mechanics can be harsh. In the Ultraxion case with ferals, it's a pretty significant DPS loss to the point where they're often seen as a liability on the heroic version of the encounter.
What can Blizzard do about it?
Like it or not, encounter design has a huge impact on game balance. The question then becomes, what do we do about it? Do we make sure that every spec has an equal option to every situation? Do we strive to make sure that raid encounters offer a true mixture of encounter types? There isn't an easy answer to the question, but it's the question Blizzard faces with each new raid.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. Be sure to look up our strategy guides to Cataclysm's 5-man instances, and for more healer-centric advice, visit Raid Rx.