The sims 3 is absolutely terrible, and completely ruined the sims experience for me. I was expecting an improved version of sims 2, but what I got was a complete mess. I can no longer tell stories because there is no storytelling mode in-game, and because there is only one saved game allowed per neighbourhood, so gone are the days of playing several families at once. At first I was happy to be able to visit other sims' houses and have my sim's workplace right there in town, but with my sims all over town, I can't watch the whole family without zooming all over the neighbourhood. Instead of complaining about the lack of improvements over sims 2, I find myself complaining about all the things that were lost. So I'll put all that aside, and pretend that the sequel to Sims 2 hasn't been released yet and go from there.
Sims 2 was without a doubt the best game I have ever played. It had two layers. First, it had the basic time management game brought over from the original - keep your sims' meters full or they'll die, then have them work on their career when they have the time. That made the game fun to play for hours at a time. Second, it had the storytelling mode. After I was finished playing all my families for 2 days each, I would take pictures and write the next chapter of my story. The ongoing saga kept me coming back to play one more round. I was still playing Sims 2 two years after I bought it.
There were few things wrong with Sims 2, but it was so important to me that I was really looking forward to the next sequel where all the flaws would be fixed. So here is a description of what I want to see in Sims 4:
First, I want the sims to age gradually, meaning no more life stages. I want smooth growth from baby to adult, and smooth aging from adult to elder. More importantly though, I would like to bring together all the stray parts into a single game. The game focuses heavily on time management for the basic motives and career objectives, but when it comes to romance, having kids, and socializing, there is way too much freedom. The sims' social, romantic and family needs should be addressed by motives as well, so your sim is required to interact with specific family members and friends on a regular basis, as well as look for romance, and participate in their favorite activities. Differences in a sim's preferences and motive decay speed would be addressed by sliders in CAS. I envision a to-do list replacing the motives panel, moodlets, and wishes, with actions becoming more urgent the longer they are ignored.