tl;dr: title
ARR (A realm Reborn) being either 2.0-2.5 or just all (current) content before Heavensward.
For me it's dungeon design and zones.
Dungeons:
With dungeons I mean any dungeon before lvl 50 (+edit: and some lvl 50 ones as well). The main thing I really like about these dungeons over pretty much any of the other ones is the fact that they aren't hallway simulators.
All those dungeons either have optional rooms, skippable mobs, interaction with environment or irregular paths. This makes them feel more alive and more like a real adventure that doesn't FEEL that scripted. I know what usually happens is that 90% of the runs just take the more efficient path and skip as much as possible, but it still just feels more satisfying and immersive if you simply know that you have the freedom to do otherwise. It keeps them from feeling too stale.
Those extra stuff in dungeons are ultimately meaningless, but there usually is lots of stuff that are essentially meaningless in an RPG but are there for immersion and giving you a (false) sense of freedom. It's all about how it feels and how you experience it.
Zones:
While at first I was excited to see the larger zones and fly around in Heavensward, it kind of got stale pretty fast and can't help but miss ARR zones now. Those zones are much smaller, but more detailed and feel more alive. It feels cozy and gives me a warm feeling when going to those zones right now and comparing it to the larger empty (dead) zones post ARR.
The game is already very instanced and nowadays I kind of wish they had just embraced it and kept zones smaller and more lively.
However one potential problem with smaller zones is the immersion and lack of space to create big (and beautiful) structures that are perhaps relevant to the story. So I'm not sure if it would actually be the way to go all things considered.
The zones aren't nearly as much of an issue compared to the dungeons though for me.
What do you think? Why would you (not) agree? And what (else) do you miss from ARR?