The major differences in the sims 3 is the open neighborhood. Instead of waiting for a loading screen you can move your sim throughout the neighborhood where they can visit friends, go to the movies, go shopping, etc.
Universal Aging. There is an option that you entire neighborhood can age together. While your playing family A, family B might have a baby, get married, or move away. Some people aren't sure if they like this feature yet.
What is said to be Ultimate Customization. You can customize almost anything and everything in the game with a platte of colors or patterns by just clicking your mouse. Sims can look athletic, while others can be obese, etc. You can even change their voices I heard.
my game wont work my computer has the requiremens and everything and the first game ddint work then we exchanged it and the second game doesent work either...