I like/love these, among other things:
- the art style, exaggerated expressions included
- that I never, ever, ever have to start a new save just to use a new feature or play in a new world; in all these years I've had only a handful of saves
- that the progression currently being added to the game is under my control: I can say no
- the CAS push-pull system, which makes wonderfully individual Sims possible
- the room moving-about options in build/buy, which saves my bad builds so often
- mod-friendliness (as a PC player)
- the lighting in Ultra
- the adorable Toddlers! and that's a someone who was not keen on Toddlers in previous Sims games
- the complexity of some of the systems added -- Vampires, for example. Celebrity. I can play these aspects of the game multiple times and it's different each time.
- how once I set it up, the Clubs system livens up the world in a way I told it to. Plus it's just plain useful for meeting other Sims.
- the rotational gameplay structure and settings are nearly ideal for me personally