Diablo 3 has been all about Demon Hunters for a while now. The thing about the game is that right now the endgame is utterly dominated by insane set bonuses. You know things have gotten a big nutty when people say "So, how about a make a build that casts big spells at the enemies?" and everybody laughs at them. The idea that anyone can accomplish anything of note by just attacking the enemies is a joke. Demon Hunters have a set that lets them drop several invincible pets that each spam the best Demon Hunter attack spells. Once the pets are down the player doesn't even have to be on the screen and they can go hang out in another room while their ludicrous pets mow down the enemies like grass.
Blizzard has decided that this playstyle is both overpowered and boring. I tend to agree on both counts, and the fact that Demon Hunters are obviously crushing other classes in achievement and popularity supports that hypothesis. However, Blizzard doesn't want to just say "We are nerfing you guys TO THE GROUND" because people will scream. Instead the latest patch notes try to claim that they are changing the offending set bonus to something else without it being a nerf. Essentially the change is that the pets only fire when the Demon Hunter fires, forcing Demon Hunters to actually attack and manage resources and such instead of just hiding.
As they are currently listed it seems completely clear that the changes are a huge nerf to Demon Hunters. Not because their numbers will go down in the land of spreadsheets but because in practice the enemies actually attack you. Having pets that do all the work while you hide is insanely powerful because it lets you ignore defensive stats and go pure beatdown. In the new model Demon Hunters are going to have to spend a lot of time attacking and being in line of sight of the enemies and that liability is crushing.
All this is a good thing in general because it will make the Demon Hunter playstyle actually respect the enemies a bit and bring them back into line with other classes. The interesting thing to me is how Blizzard is trying to present this as a neutral change and avoid the perception that it is a big nerf to Demon Hunter power. There is a huge amount of politics involved in this sort of thing because players do appreciate it when their favourite class is competitive but once they have switched to the flavour of the month they get pissed if it gets nerfed TO THE GROUND. I think Blizzard's strategy is a good one though ultimately I don't know if it will work, or indeed if anything would.
You can claim that their changes are neutral when the opponents are trivial but everyone knows that after this happens there will be a mass swap away from Demon Hunters to other classes because their absolute dominance will be at an end. Toughness and mobility matter and when a change suddenly destroys your ability to stay alive it doesn't matter if your theoretical damage is the same. The conclusion that this is a nerf is inescapable.
Sometimes though you have to do the best you can even when you know you can't win. I suspect Blizzard is in that boat right now and they know it. They will claim these changes aren't a nerf to preserve plausible deniability and go ahead and do it because it is the right thing to do.