I'm not normally one to go down the cheesy road of posting these kinds of things but I really feel compelled to. Heavensward is incredible and leaves 2.x in the dust.

-Every new zone I unlock is impossibly more vast and impressive, and beautiful to look at than the last.
-Same with the dungeons, they just get increasingly more incredible. Last I did was The Vault, reminds me of Dark Souls - enemies and all. Bosses were a nice step up too :P
-The MUSIC. Holy crap. I don't even know what else to say here, me and friends have been humming various versions of the Dragonsong theme for days. The Bismark music isn't what I expected at all but in a great way. Definitely a standout feature, Soken poured his heart into this and it shows. And not just the music, all the sound effects like the thrilling "whoosh" sound when landing from flight :P
-The story. 2.X is nothing in comparison, it also truly feels like a main FF storyline. For an MMO to have a plot of this quality is something to be proud of.
-The difficulty. Apart from Coil, not much in 2.X really felt taxing enough imo. Now the wild enemies are a lot more hardy and mean (even those cute fluffy ones in Sea of Clouds), and the dungeons - so far - feel just right. Also with all the new skills it feels hard to call any job brain-dead easy (though they do vary between each other, of course).
-A few data center issues, but honestly it's been a relatively smooth launch!

There's a plethora of sidequests that can get very tiresome, but at least they're more streamlined than in ARR. I feel I'm a lot more likely to remember the hunters of Dravanian Forelands than...whoever was around Western Thanalan.

Generally, you can just tell that the dev team absolutely gave this expansion their all, and the quality dwarfs ARR so much. I just wanted to take some time out of my 4-day no-life marathon to acknowledge that work. [/cheese]