I would like to preface this by saying that I am not a sims person. Somewhere I have a copy of the original sims, that came free with a computer I bought long long ago, that is still wrapped in plastic. I never even looked at sims2. That being said, I really like sims3. Really.
Back in the late 80s and 90s I used to play computer games that were turn-based multiple charater fantasy RPGs where you basically had a team that you created and controlled that ran around and saved the world (or solved the quest, or rescued the king, or whatever). I liked these, they were fun. They were also very linear, which I found a bit limitting, and they went out of style some time in the late 90s (there is a newish one on the market called Drakensang which is fun). My favorite was Darklands, which was very non-linear and was set in 14th century Germany. Darklands let you control the story. You could wander around and develop skills, stay in one place and earn money, or quest around and find adventures, so the character stories went in different directions despite the rather primitive graphics and limitations of the day.
But Nic, you say, you can do that in those online games. Well, yes, but you can only control your one character at any time. Also you have to interact with people and I do enough of that during the day, if I'm playing computer games it's because I'm tired and brain-dead.
So, I like free form and I like team based and I like the option of turn based. I also like fantasy, but ya can't have everything. Sims3 does this. You can play up to 8 characters at a time who run around town doing whatever (um, I have no idea if you could kill the NPC sims, it isn't something I'm interested in). The game is exceedingly free form, marginally self controlling so that you can leave one or more of your characters alone, and you can make it stop while you think about what you want to do next so that you have good control over the action as well. You can also create your own goals and story with the characters lives and you have control over their skills development. It's fun, I like it.