I've played a ridiculous number of hours in this game (over 3k, I'm pretty sure) and here are some, but not all of the reasons:
The managed households/manage worlds function - which is probably the most underrated feature in the Sims 4. I love how easy it is to edit, delete, add, change, ect both townies, npcs and played families. Pulling a lot off of the gallery is super easy and so is plopping lots around, moving them around, editing them, etc... It was so clunky in previous games, and now it's easy and fluid, and as a rotational player who likes to play neighborhood wide this is a huge, huge thing.
CAS - It's just so easy to make interesting looking sims now. It's lightyears ahead of the previous games. There is really no comparison.
Art Style - I think this game (especially using reshade) is freaking gorgeous.
Animations - the sims themselves and their facial expressions are so much better than previous games. I used to play the Sims 1, 2 and 3 from the top down or zoomed out from This time around I find myself playing and watching much closer because the sims are so expressive. It's great.
Build mode - I'm a terrible builder, but even I can make nice looking houses in this game. It's so much more intuitive and easy now, especially with the dragging functions. And I love how I can just pick up a house and flip it around or move it three tiles to the left without having to tear everything down now.
Game performance - I have a ridiculous amount of mods and cc crammed into this game and yet it only takes a couple minutes to load and a few seconds between lots. It's so smooth and reliable that I really think the team doesn't get enough credit for all of the work under the hood they must have done to get this game to run so well. Don't have to worry nearly as much about file corruption, game breaking lag, routing issues, and completely broken expansion packs etc. The Sims 3 was a freaking nightmare for me, and so I really love how this game runs.
Custom Content - there is an amazing amount of CC and mods for this game that are really, really great. I mean, I've been tossing cc into my games since the Sims 1. New cc always helps keep things fresh.
