The thing here is: For what kind of player do you want to make the game harder and do these kind of players see any problem with the current system?
For I always see people who do not play high-end and see their game focused on the leveling, low level dungeons, campaign questlines etc. complain about how easy the game is. But you could also just tell them the lack of game-insight.
Just as the pro-vanilla people are playing the game entirely different from the current top 1000 raid guild people. There is a huge gap in philosophies how to play the game also coming from how blizzard designs the game. When raid mechanics and tuning, pvp tuning and class tuning takes so much more "resources" as in "dev interest" (game support) than anything else, it suggests a certain group to be more on "bright side".