Exploration
I'm a huge fan of exploration in games and Inquisition definitely hits the mark for me in that regard. I love how varied the locations are and the environments look amazing. The Emerald Graves is probably my favorite area - I ignored all the quests just to wander around staring at the lovely trees for like twenty minutes.
My second favorite is Val Royeaux. It's like stepping into an old painting and is ridiculously beautiful down to the gold leaves sprinkled by some of the doors.
Also, unlocking new areas as part of war table missions is super cool! Specifically, I remember looking around Emprise du Lion and noticing these Colosseum-style buildings in the distance and thinking to myself how amazing they looked and how I wished they were more than just background art.... Then I happened upon the broken bridge you can fix with a war table mission and realized I could actually get there. Freaking awesome.
Last specific example: I immensely enjoyed the 'haunted' Villa in Emerald Graves. When the first lamp lit up by itself, I was all freaking out to my computer screen and Cole said something creepy like 'it knows we're here'. I love Cole so much, but he is not the party member you want in a horror-style setting. What do you mean it knows we're here?! Ha ha, loved it.
Heroism
I really felt like a hero playing the Inquisitor. I know some folks aren't so into the 'go kill 10 rams' quest, but I really liked knowing that even small tasks were part of trying to make an actual difference. And that difference is shown both by how other characters respond and also by the environment changes. It's a pretty good feeling to quell the mage/templar fighting in the Hinterlands and then those random fights don't happen anymore. Sure you'll see red templars or something else pop up later, but it still demonstrates player impact.
The whole sequence defending Haven and getting to Skyhold felt incredibly epic. Same with the scene before the Arbor Wilds. It was cool seeing the forces you'd recruited gathering up. On the way to the Temple of Mythal, I felt so proud of the mages standing on a bridge fighting the red templars below. Yeah, those are my mages!
On a smaller scale, after the Crestwood resolution, I suppose having the dawn break and the rain stop is a little cliche, but I still loved that it happened. 
Characters
One of my favorite things about the Dragon Age series is the interaction with the companion characters and Inquisition does not disappoint. The dialog, personal stories, and voice acting are top notch, as always, but what I didn't expect was how much the companions surprised me.
I'd sort of written off certain characters in the beginning like Blackwall and Vivienne, and was pleasantly surprised (or horrified) as more of their personality and history were revealed (we are not even going to talk about Solas). This made the characters a lot more interesting and part of the reason I was excited to do a second play through of the game. Hearing Iron Bull basically call out Blackwall from the beginning in a party banter was awesome.
Also, I like that there is more to party banter than just flavor, especially if Cole is there, and that the relationships between the characters may change based on the story events that take place. For example, I loved that Solas and Iron Bull develop a kind of friendship if Bull becomes Tal-Vashoth. Their chess match conversation is one of my favorites.
Non-verbal Storytelling
Not sure if it's the new game engine or what, but I found myself paying as much attention to character body language and facial expression for communication as the actual dialog and voice acting. The specific example that jumps to mind is a bit heart-wrenching, but the way Solas's eyes become sad just before he breaks the Inquisitor's heart into tiny pieces was incredibly well done. Also: 
Another example I remember is when the Inquisitor meets Sera for the first time, one of the dialog options is something like 'You sound like a thief acting out petty revenge fantasies' and Sera's facial expressions in response are awesome.
Hidden Stuff
There is a ton of lore information in the game for the player to discover. The unreadable codex entries revealed after drinking from the Well of Sorrows is fantastic. One fan recently found hidden backwards audio messages during some of the scenes. It's super fun speculating about what is going to happen next and discovering info that was purposefully placed into the game.
Girl Power
One of the reasons BioWare games have remained among my favorites for years is that, as a female myself, I can play a female character and still feel completely immersed in the world. There are no awkward pronoun mishaps in the dialog or an unrealistically proportioned, scantily-clad woman who's obviously meant to be the 'real' love interest.
Seriously, I know some folks roll their eyes at the romances, but I still think having any content not specifically geared for straight men is pretty amazing.
Also, it was a wonderful feeling to look around the Inquisition war table and see a bunch of strong, amazing women in positions of power. Cassandra, Leliana, and Josephine aren't there as window dressing, they are there because they are capable. So thank you! 
I could go on but this is too looooong already! Thanks for an amazing game!