I'm gonna be straight-up honest, the ability to make bona-fide "crazy eyes" is my favourite thing so far.
There are certainly plenty of opportunities for creating, ahem,
interesting sims. Drizz must love it
.
So far, I have to say I'm not that knocked out. It's a lot more intuitive than the Sims 3 interface but it doesn't seem to be as flexible: the Sims 3 CAS in advanced mode allows you to manipulate things that you don't seem to be able to in the Sims 4, at least in the demo version: the area between the nose and top lip, for example. Those lines between the nostrils and lip - can't remember the correct anatomical term, if I ever knew it - seem to be pretty prominent in 4 whatever you do, whereas you could make them very obvious or shrink them to nothing in 3. Maybe I just haven't worked out how to do it yet.
The lack of clothing choices is also a little irritating, although I realise that is likely to improve. For example, the Tshirts for female Sims: why do some colours only come with a picture on the front and others only come as a plain shirt? Offering both options for all colours would be a tiny overhead in terms of programming and disk space.
Funnily enough, CAS-obsessed daughter (who spends at least 95% of her time creating Sims rather than playing with them) hijacked my computer for a couple of hours this morning and had exactly the same gripes as I did.