Anyone who played since then knows how much Quality of Life we've had since then. It's easy to forget just how much we've had, but if any of us went back to then, we'd miss all the Quality of Life and want to return to now.
Contrary to what you see everyone say, most of what SE has done since ARR has been improving the game.
There is far too much Quality of Life for me to even remember off the top of my head, but they include things like cooldown resets upon wiping, not needing to level another class to 15 to unlock your job, not needing to level other jobs for basic role actions, removal of bloat like Featherfoot and Keen Flurry, action/ability upgrading so we don't have to make macros, cone targeting, disable right-click selection of minions and players in battle, hide quest list during instanced duty, alliance raid roulette, normal raid roulette, cross-world PF, [Duty Complete], [Duty Complete (Weekly Reward Unclaimed)], [Practice], PF search, sounds when a party fills, world visiting, DC travel, having 999 stacks instead of 99, being able to teleport to a dungeon boss arena after it locks if you didn't get there in time, listing teleport destinations by expansion, PLD having aoes, tanks having aoe combos, tanks physical mits now mitting everything, not needing to face a target to auto, not needing to face a target to block, ranged attacks not resetting your combo, combos taking longer to expire, buff timers displaying in the party list, waymark saving, more waymarks, mount speed increases (we used to move super slow due to PS3), chocobos no longer in the party so you can queue for duties with it out, double the max food and gysahl green duration, flying in ARR and all mounts can fly, a 4000 cap on hunt currencies instead of 1000, old tomestone gear is pre-augmented, mount and minion favorites, New Game+, unlock quests being blue...
There are a few things people miss from these expansions. But it really is just a few things. Most of it would be a nightmare if we went back. And it's not that they really want things to be as they were then, but rather that they want things to be punishing or sophisticated in the way they were then but not in the same way.
For example, there could be more complex substats... but I don't think people really want them to be like Heavensward like they claim. Because then we had parry, an RNG stat for useless physical mitigation; we had physical mitigation like Foresight and Sheltron and Raw Inuition, elemental mitigation that was useful against only a specific cast in specific fights and therefore useless, and Accuracy which you had to stack to 100% or your combos got interrupted all the time. The substat system was more complex, sure, but not in a good way and I think people mean to do it in a "good way" rather than to copy Heavensward.