The natural evolution of the Sims series, "The Sims 3" adds more content and streamlines gameplay, but is basically more of the same.
Like previous "Sims" games, The Sims 3 is about running a household of individuals with different traits, tastes, and outlooks and either making them prosperous or tormenting them for your personal amusement. The basic formula hasn't changed much, since it's still generally based on fulfilling Maslow's hierarchy of needs (food, hygiene, social contact, etc). The individual components of a Sim's life have been expanded somewhat, however.
A Sim's mood is managed via "moodlets", short-term status effects that show immediate wants and concerns as well as positive influences. While the traditional six attitude bars still exist, managing a Sim's needs is streamlined through the implementation of these moodlets since all you need to know is right there in front of you. The AI is pretty good at managing moods, too; as long as enough facilities exist, you can basically leave the game running and your Sims will take care of themselves. While their timing might be a bit off, Sims will respond fairly quickly to their needs and don't need constant prodding and micromanaging to do what they're supposed to do.
The career system has been improved in a few ways. For example, it's possible to do different things while at work (previous games essentially had the Sim disappear for a few hours, then come home with a paycheck). These are limited, but include things like working more or less hard, or choosing to build relationships with your boss or co-workers. It's not much, but it's a neat inclusion. There's also Opportunities, which are random assignments based either on your career or your skill-sets that can reap valuable rewards.
The spice of the game largely derives from Sims' different attributes, whether they're social like "Friendly", "Flirty", or "Evil", skill-related like "Athletic" or "Artistic", or just traits like "Tidy" or "Lucky". These affect both a Sim's behavior and their available dialogue options. Since traits are picked at character creation, it's really up to the player whether their traits are conducive to their preferred type of gameplay. If you want a safe, controlled household, pick positive traits. If you want things to go wrong in a hilarious way, pick negative ones. In the latter case, it's usually more fun to let your Sims do their own things, because with the proper traits they'll go ahead and ruin their lives without your help.
Overall, the Sims 3 is full of lots of little upgrades - and while that may not be a problem, I don't think it's enough to really bring in anyone who wasn't a fan of the old games. The basic concept of "run some people's lives" is the same, and if you like that then Sims 3 is better at it than previous Sims games. Like the Sims 2, there's a lot of extra content and DLC (the basic Sims 3 package only delivers standard content), but again none of it's really worth it unless you're already into the basic concept. The best way to put it is "If you like the Sims, you will like the Sims 3". It's got better gameplay, better graphics, and more stuff, but it's more of the same.
We purchased this game with our own funds.