I think it's safe to say that this game is either using The Sims 2 engine, or is reusing a lot of code from that engine. When it comes to stability, I think that's a good thing. However, it does worry me how long the loading screens will be once there are expansions and a lot of custom content installed. Time will tell.
So my pros and cons:
Pros:
- (Potentially) much improved stability for the years to come.
- Improved routing.
- Bringing back the multitasking from the Sims 2 (people seem to forget that this feature once existed).
- I don't build much, so having droppable rooms is good on the occasion that I do.
- All new animations (people also seem to overlook the fact that the animators in the Sims 3 were pretty lazy there and reused a bunch of stuff from TS2).
- The graphics.
- Overall fluidity and realism.
- The UI.
- Unlockable features and items.
- The search feature in Buy Mode. (Why, oh why did they never add that in TS3?) It will be interesting to see how that interacts with custom content.
- Emotions.
- Food that can be modified (such as cake).
- The ability to change clothing and makeup/hair all in the same place (without having to have a special stylist mirror).
Cons
- Lack of story progression (although no doubt that will majorly improve stability as well).
- Lack of toddlers (I'd rather they exist and be non-playable than not exist at all).
- Inability to switch Sims while on another lot.
- Inability to see your Sims go to work (though I also hated rabbit holes).
- Lack of Create-a-Style
- Lack of a repairman.
- Teen height. I'm having trouble telling teens and adults apart.
- Relationship icons on profiles are more subtle than in TS3. Sometimes i have to squint to differentiate friends from acquaintances
- The Sims 3 would only let you right-click on a visitor's profile to center view on the visitor, but would not follow the visitor. The Sims 2 let you keep the focus on the visitor. The Sims 4 has neither option. Please bring back the ability from the Sims 2.
- The ability to change aspirations at any time or change a Sims' body or traits (although I don't have to use these options, so I'll admit that complaining about them is kind of nitpicky).
- Not being able to tell if a neighbor's house is vacant until you sit through a loading screen to get there.
Overall, I like the features that were brought back from TS2, but I would rather have seen a closer hybrid of TS2 and TS3. I'm not really going to gripe about the missing objects or features because I understand this is a base game, and am, frankly, old enough to remember that The Sims (as in TS1) didn't even have parties. As a side note, I feel that the Sims 2 made many core additions through its expansion, and I'm hoping that means that the Sims 4 engine will be much more flexible than TS3 ever was.