Giving monsters unique gimmicks like the Mordrem did in LS2.
There wasn't anything particularly special about the Mordrem though. The Forged are just as new as the Mordrem in my book. You have light, medium, heavy, super heavy enemies with a variety of new attacks, new spells, new skills, etc...
Making meta-driven maps like Dry Top and Silverwastes and then expanding on that system in HoT.
One of the biggest criticisms of HoT were the map metas. The prevailing opinion was that the HoT maps were 'theme park rides' and not exactly proper maps with proper exploration; this was especially true of Dragon's Stand. Honest question for you: how often do you go back and play Dragon's Stand? Once I finished all the collections, I never looked back.
Picking a theme for a new map and running with it, like the extreme verticality in Bloodstone Fen, the grappling hooks in Draconis Mons...
I like that the new maps don't have a 'theme'. That's part of what made HoT maps just seem like one trick ponies with a 'theme park ride' feel to them. When the meta wasn't going on, there was little to no real life in the map.
The new PoF maps seem like 'real' places the way core Tyria maps are meant to. You have NPCs doing NPC things. There isn't a theme. It's a map with events and hearts going on and you explore it for the sake of exploration the way you did in core Tyria. I like that.
...and the way Siren's Landing used it's hearts as a gating mechanism for a daily reward.
The new hearts I believe are what allow you to ID unid gear. Gives reason I think to repeat hearts.