To begin this is not a bashing and bashings aren't allowed on this forum so please do not reply with it. If you want to discuss the difference between Sims 3 and 4 there is a topic located
here.
The point behind this post is to help to explain some of the changes from my knowledge of things (I went to school for graphic designing and did some gaming classes as I was debating doing video game development). I wasn't going to post this but my partner encouraged me to after I explained some of these things to him.
Some of the common things I've seen around here and the web...
Lack of the Color WheelA color wheel has about 16.77216 million colors available on it. Each time you used one shade different on an item the game had to create a new item in it's files in order to produce it every time you saw it. This is why after playing a game or large families you began to see stutters. There was nothing the game developers could do about this.
Cartoon LooksIt is actually common for game designers to use a cartoon look to their games. The reason behind this is it let's the game remain timeless and not look outdated over time. I know that sims are a simulation of real life but take a look back at anyone in the 50's, 60's, 70's, 80's, and 90's we look dated too (even though I too rocked the spikey hair and neon clothes in the 80s). This is a good thing from a development standpoint that means they are thinking of long term for the game and won't hit the limit of looks and ideas any time soon.
Instanced AreasAs you know loading all around you takes more and more the more of a world your loading (I think this one was general knowledge). The parts that complicate it was the amount of people and having them all function while still functioning your sim and family. Sims can become stuck or glitch and it will kill your game. Things left on cause more and more to processes. There is also a lot of coding and weighing when you try to balance which of these sims show up in public lots when you visit and which continue with their random day to day things. When you jumped in a car and traveled to a location the game had to process every sim you passed along the way. Although it may seem silly to have to "travel to your neighbors" it's so that wherever you are is all the game cares about making you #1. There is actually a good amount of things to do on the areas so I think they found a good balance. From my experience this was also a trade off to allow you in the future the possibility to travel to any expansion later released by using the map (I can not confirm if that's what they will do as they haven't released one but I see that as a definite potential). The layout and design of the map shows the ease adding new areas would be with the circles. So you would no longer be stuck to only living and visiting places in your world down the road.
Lack of Pool and Terrain building *We have pools now*There is a reason I placed this after instanced areas. Although many don't know it when you have a game even if all you can see is the ground if it has a lower area it must process that area (on the off chance it might be sloped or modified). This means if floor to sky its 300ft the game must process that but if you have 300ft down you can go as well then the game must watch 600ft of space. This adds to a lot of accounting for things. Now pools add moving water and usable items and a whole slew of objects/movements that must interact differently. A tub you are stationary a pool you have a set of moving objects and actions happening. With Sims 4 there is multitasking which would say you should be able to in a pool as well. I am sure that added tons of obstacles coding and effect wise.
I hope some of explanations helped to clear up things for some people. If anyone has anything they'd like to see if I have an answer for or need more clarification or examples I can try my best to help.