I am both amused and annoyed by "But Blizzard does it all the time, why are you suprised?!".
Actually, they don't. Yes, of course there are nerfs every major content patch - and numerous hotfixes, too. But let's look at the scale of it. Pretty much everything has been nerfed by 20% - that's almost Sunwell scale right there. In addition to that, numerous skills have been further tweaked, in some cases damage/effects got halved, meaning total comes out to more than 20%. It's pretty much Sunwell patch, except done "mid" expansion. Changing entire tier into Naxx - which they even confirm is their intention, with "introductory raid". The question is, how easy things must be to consider them "introductory" - and what comes later, once "introductory" raid has been completed?
But that's fine, it's old content, and being 13/13 it doesn't affect me. I just don't accept the "nerfing bosses by 1/3 is normal practice that was hapenning since always! I am so wise and great for pointing it out, even though it isn't true". So far it has only happened with Sunwell, and this is the second time. Even ICC buff didn't happen instantly - plus it didn't change any mechanics, just allowed to power through them with more hp/dps, which is happening anyway.