It's pretty good!
The newest and biggest addition to The Sims 2 is aspirations. Basically, every sim has an aspiration (a purpose in life, a profession, something to do), and they have wants and fears. To succeed in the game, you can fulfill every want they have. Or, if your the sadistic type (all of us), you can realize all of their fears and make them go insane. When you realize their wants, you get aspiration points. You can spend the points on certain objects that can benefit or ruin them in many ways. The aspiration points system can get a little bit annoying at times though.
Maxis has improved build mode by adding a lot of more building options. You can now add foundations to your house, connecting stairs, and even multi-level houses if you're skilled enough.
The buy mode has also changed. There is this new design option that lets you alter the appearance of some furniture. For example, if you buy a sofa but don't like the colour, you can change the colour if you pay an extra 15 simoleons (sim money).
Another very noticeable improvement is in the graphics. The graphics have had a major overhaul and The Sims 2 actually looks like a new game. Sims have gained a lot more polygons. They have articulated fingers, hair that blows in the wind and real teeth.
The sound usually consists of the usual gibberish spoken by the little sims. The music is really varied. It has a wide selection of music tracks but please, please stay away from the Pop tracks. They're awful! Well, I think they're awful.
As with every game, The Sims 2 has some flaws. Sims still stand around doing nothing if all their wants and needs are full. There are also some annoying glitches but they will probably be fixed in later patches.
Plus: Aspiration, graphics. New build options.
Minus: Aspiration, old pcs might not be able to run it. Awful Pop tracks. Glitches. If you didn't get it the first time you still won't get it this time.