Overall? Hard to say. WoW used to be harder, but much of that harder was because of terrible balance, not due to actual challenges. Only one real tanking class made it harder to do raids due to lack of tanks, but fixing that is hardly what I'd call a nerf.
There's not much to be gained by tallying nerfs and checking the score. If the game is too easy, it means you're not trying hard enough to make it hard. If you're not willing to put in the effort, don't complain. Maybe this is where WoW has been nerfed: challenges are handed to people on platters. There didn't used to be specific challenges spoon fed to raids.
If they wanted a challenge they made it themselves. I didn't see that happen often though. Why not? Why do most people not do something unless the devs tell them to do it?
My last paragraph is most relevant: Why don't we make our own challenges?
Perhaps this helps to explain it: we get banned. But there was a lack of such initiative before this. This isn't the cause. Why don't people make up their own challenges?
On my first wander over to Yggradsil I came up with this: (it makes a little more sense in context, but I think it still works)
I have noticed that spending less time often results in more fun. Oh sure now and then it's nice to lose a whole day, but for the most part WoW fun has diminishing returns. It starts to get stressful, especially if you spend half your time staring at the LFG interface thinking "I don't need that heroic, I don't need this other heroic, there's nothing to do."
It's not just the time though. It's what you think about the time. Is the time work or is it play? Too often I think we start seeing it as work, not rewarding in itself but only for the rewards. When that happens it's pretty obvious why we'd get angry over content getting easier. On the other hand, if play was purely for the fun and experience, then we'd pity those who come after the nerfs because they won't get to play what we played.
We're motivated by gear and other tangible (as much as virtual can be tangible) rewards, not as much by experiences. Self-made challenges do not give rewards. The devs don't intended for us to do them, so they don't reward them either.
What can change?
Ever heard the phrase "not working as intended"? That's another way of saying bug or exploit. The four man kill was not working as intended. The devs wanted the fight done with 10 or 25 people, not 4.
Change the philosophy. Rather than having just intended (do this) and not intended (don't do this, or else), add something else: unexpected (that worked? Cool!) I realize this makes things difficult for the devs and GMs. Where is the line between unintended and exploitative?
I hate to say it, but City of Heroes/Villains might actually have something to contribute. As far as I can tell from secondhand evidence, the game doesn't really have intention or balance. The result is that people just do fights however. My friend claims it makes them think, I thought it made them never have to plan ahead if just anything works (meaning it goes to the other extreme of non-thinking). A middle ground could be cool though. Take current raid design, loosen it slightly, and ease up on the bans because something was "not working as intended."