There have not been any across-the-board nerfs to Heroics, let alone any changes made due to complaints about difficulty.
We're happy right now with the overall difficulty of Heroics, but that doesn't mean we won't make tuning adjustments to certain encounters or abilities if we feel they're unintentionally over- or underpowered. That's what we've been doing in our hotfixes recently. Here's an example:
- The Crystal Shards creatures in the Corborus encounter are now level 86, down from 88.
[...] And I'm saying it's unfortunate to view difficulty in such a black-and-white way. We've also made changes that affect difficulty in the other direction that I wouldn't necessarily call buffs.
- Ripsnarl's Summon Vapor mechanic now ignores line of sight and correctly spawns a Vapor in regardless of where players are standing.
Players were able to bypass Vapors using line of sight. This was fixed.
- Impaling Slam by Forgemaster Throngus now does much less damage, but is no longer mitigated by fire resistance.
Yes, this was a reduction in damage to Slam, but damage was reduced so that we could change the ability so it ignores all resistance modifiers, which makes it easier for us to keep the ability tuned how we want.
- The "corpse-catching" mechanic now properly teleports players to the entrance platform of the dungeon.
This didn't necessarily buff difficulty, but it was a change to prevent players from skipping certain trash pulls.
- Troll druids in particular, when shapeshifting into Cat/Bear form, were dealing an extra 5% damage to themselves via Beast Slaying when afflicted by Barion Ashbury's Asphyxiate. This caused them to die instead of being taken down to 1 health. This has been corrected and Asphyxiate now properly ignores damage modifiers.
This one doesn't really reinforce my point, but I thought it was a slightly hilarious issue.
We've also removed experience, reputation, and sometimes loot from several creatures in dungeons which were too easy to farm.
Phasing in World of Warcraft, be it normal phasing or terrain phasing, plays a heavy role in questing right now. Accepting quests, completing quest objectives, turning in a quests: these actions can all move your character from one phase to another for the purpose progressing a specific story.
For clarity, terrain phasing is very similar to normal phasing (which hides or reveals sets of creatures, intractable items, NPCs, and buildings); however, instead of just allowing us to adjust objects in the world, terrain phasing actually gives us the ability to change the world itself -- or, more specifically, how it appears to you. Despite these capabilities, we don't currently use phasing to moderate population levels or arbitrarily isolate characters within a certain level range from the rest of the community *. It's simply a means for us to provide our players with more dynamic and engaging content.
* Goblin, worgen, and death knight starting areas could be considered an exception, given that Kezan, Lost Isles, Gilneas, and Acherus are all separated from the world-at-large via phasing. This is due to the nature of those starting experiences, though, and not out of a desire to restrict player interaction.