I guess that my own personal grip with older content (speaking about dungeons mostly now) is the downscaling and restriction on usable skills.
After spending a lot of time at higher content/end game the restriction of skills do make the content I do feel more sluggish, even if it isn't compared to when I played it while leveling.
And pressing the same 2-3 buttons over and over instead of my whole hotbar isn't the most fun.

But I can endure it.
I just don't find it as fun as higher content where I got everything at my disposal.