IMO when we are talking about difficulty of normal content we are really talking about difficulty of dungeons. Open world content is largely unused after the first month or two of a new expansion, and the story is solo and the number of real "fights" you have is minimal and not content your doing on a daily basis. That being said, I think dungeon content suffers from a few design flaws. The first and most prominent is the healer role. For the other two roles dungeons are some of the easiest content in the game. Tanks just have to have tank stance on and make sure to hit all the mobs and Dps just literally have to not go afk while hitting buttons. For healers on the other hand, healing big pulls is one of the more difficult things to do. It is much easier to heal in raids and alliance raids then healing a big pull. If you are new to the role/job and can't keep up the consequence is that everyone dies and is likely blaming you for why their run is taking so long. This results in people trying out healer with "easy" content like a dungeon and then when they fail miserably they stop, ultimately resulting in less people who like to play healer. The second issue is there is no real feedback for players of the other two roles. Think about this for a second, how do you know that you did a good job in a dungeon as a dps without a parser and some knowledge of the average damage of your job? The answer is you don't, all you can use to judge your performance is the speed of the clear and how many times you died, neither of which is a good measurement for your skill. You might clear fast in spite of your performance, not because of it, and you could be dying to mechanics while actually doing your rotation correctly. With no real incentive to push dps players, most get content with doing what is comfortable and don't really improve until they start doing fights that require dps checks (end game), and even then they just opt out of that content because its "too hard".

Dungeons are supposed to be easy, especially for veterans. They are meant to train you in jobs you may not be familiar with to get you ready for endgame content. Roulettes are an incentive for veteran players to carry new players or players learning new jobs through content.