So if a lot of dif.modes isn't content, then you should compare the easiest difficulty for particular “content” for adequacy of your discussion (precisely when discussing content difficulty), but not the most complex one. It's normal vs LFR and now think about what is it for Ulduar. Do you both understand this?
But if you want to compare progress difficulty, here is a smarter maze, because see the quote: now "all content is consumed easily through easiest difficulty, and progress, which's indicator are equipment(ilvl)+AP, isn't attainable in principle by virtue of mechanics (process is accessible to all regardless of ability and experience, since doesn't require any such stuff to participate (=borderless in terms of content), but is unachievable for anyone)". They simply missing development "ceiling" in current design (not only for a particular player due to limited time and opportunities/abilities, but in general for all).
There is no end for "progress" for anyone and since there is no end,
so there is no goal to reach that end, which was (still is! I'm not talking specifically about this game) main playing motivator for most people.
That is, developer’s trick isn't to make it unattainable in principle, but so that it can be reached by average at a certain time defined by devs (even if this time is transferred to next expansion for some and for some in advance of this deadline), and concepts of progress and content shouldn't be separated here, their division is artificial and vicious for this particular game.