I started playing the original Sims round about the time the second expansion pack came out. It's one of those games that can pull you in so much that you forget to take care of your own needs. In fact, it's a running joke in the Sims fan community.
This sequel is everything the first was, and so much more. Now we've got genetics, and aging, which means your sims can be related as more than just child or married. You actually get grandparents, aunts, uncles...all that great stuff.
Your sim can be abducted, so be careful when watching the skies through your fancy telescope. And if your sim dies, there is a chance they might haunt the family who lives on the lot they died on (so long as their tombstone is on that lot).
It's graphics intensive, but the new behaviors and genetics make it more than worthwhile if your computer can handle it. I highly recommend it.