There is so much to do...where to begin?
The number of ways you can turn your Sims is endless. I particularly like the setting where you may decide how much long will be the life of your sims. Because I don't have much time to play these days, I set it up short so that I will see my familly tree get higher in a shorter time. So after 15 hours of play I'm already at the third generation of my familly and although it doesn't allow me to experiment fully with life goals and things like that, it allows me to see different path for different style of plays. I've even pass a good portion of my time playing just building my house which was so fun by itself.
That is where The Sim 3 excel. It adapts to your game play style and it keeps pushing you softly toward a goal that you have partly set but where there is sufficiently unknown elements to keep you interested.
Interestingly enough, a game can be pretty dull if you just keep following simple goals much like life can be pretty dull if you keep your Metro-work-sleep routine all week long. But once you try different things, the game shines and allows you to experiment.
I am not of the "online type" so I haven't experiment with the exchange portion of the game but I'm guessing it is yet another dimension of the game.
Hey you hardcore gamers? Drop the facade and try this one. It may not be as glamourous to say you've played the Sim 3 instead of Dragon Age Origin or Fallout 3 but believe me, it's the same fun