I think the game looks fantastic (I run ultra minus hairworks on an R9 290) and looks fantastic without needing 150 mods to replace every asset, animation, etc in the game years after release.
Loving the content - even the most seemingly trivial side quest is often highly detailed and deep and very enjoyable.
Definitely exploring is awesome - visiting every ? on the map but more, since there's plenty of stuff out there to find that's not marked already. Definitely pays to explore both for fun stuff to find and for great views.
Huge variety of content types and out of all the stuff I've done so far very, very few quests felt like a "kill ten" or "collect ten" type of quest and even those were well disguised and usually spiced up. One example is when you're clearing up rats with a certain friend and you have to mark 3 nests - kind of a collect X quest - but there's dialog thru it and it's a quick and painless thing that morphs into something more important so it doesn't feel like routine nonsense (as quests almost always feel like in DAI, for ex).
Almost no quests have felt contrived or unfun to do, even ones where when I look at the description or start them I'm thinking, oh no, this is going to be cheesy, then it's far from that. Just an amazing job on content. You may start a "oh help me my son/daughter/wife went into the woods and is missing" quest and think, here we go again, but there's usually some twist or interesting element that makes even such a routine type of quest be very enjoyable and different.
The horse riding is very well done. The water/swimming element is very well done, a bit silly to be able to one shot mobs with the xbow while underwater when it does no noticeable damage on land to those mobs but it's better than having underwater sword fighting.
Gwent rocks - one can easily get absorbed into that.
I like that the boxing is more like regular combat and not random key spam nonsense. I've yet to see ANY of the key spam nonsense like there is in Witcher 2 and I'm glad for that, big time.
Nice varieity of mini games (Gwent, horse racing, boxing) that fit in well enough with the overall game atmosphere and give you something different to do on top of the huge variety of quest/event/exploration/main story content.
The combat feels a little more fluid than in Witcher 2. I didn't finish Witcher 2 or even get that far into it (finally got into it at all after numerous attempts shortly before 3 released) so it's hard to compare, but I like the mob behaviors and animations in Witcher 3 - blows away so many other games.
I'm up in the air over changes to how potions and oils work - I like being able to use them during combat as the prep was a bit clunky for me in 2, but, the UI is so horrible for managing potions and oils that it's no less tedious dealing with them in 3. I'm hoping they clean this up a bit.
I like how the revamped talent/skill system works.
I like the freedom, which isn't unique to this game and its open world approach but it's still worth mentioning, since as with Skyrim, or Dying Light, or other games with lots of fun stuff to do, you can easily spend hours having a blast without every touching the main story quest or even doing anything "serious" content-wise at all. Sometimes I just ride around checking out landmarks and doing whatever comes via exploring. Love it!
This game eats time in a big way. It's so easy to get wrapped up in whatever you're doing and endless distractions. Or if that doesn't suit you then you could always pound the main quest or whatever. Freedom and lotsa choice.