On the subject of challenge, and this relates to what Nadira mentioned with the end game in WoW's Cataclysm expansion, is that once the difficulty bar is lowered, its difficult to bring it back up again. Even within patches themselves, increasing item level makes it progressively easier to kill everything, and when an expansion shows up to reset all that, people don't like it. It's as much a good reason to stick to a set of standards throughout the game's life for content difficulty as it is avoiding too much stat creep.