I was browsing through another thread here that was all about how the game is "too easy" now. It made some good points but didn't really offer any insight in how they should put some difficulty back into the game.
First I think you have to define difficulty. For some people it means inaccessibility, for others it means having their actual gameplay skills pushed to the limit, and still others consider it a matter of time and effort. Is it a combination of the three or some variation on them?
So tell me how you define difficulty in World of Warcraft, then say what you would do to turn it up a notch.
For me I think it's about gameplay skills. I've been wanting something like Proving Grounds and/or Brawler's Guild for a long time and I think it will be a great way to really test that. But it's certainly not the only thing that adds challenge to the game.