All these new worlds offer new stories and new in-game characters that make them worth playing. Riverview is the only world that you can download without paying. For all other worlds, you will have to buy SimPoints, or Sims 3 store currency, to download them. The cost ranges from a $19.99 bundle, up to 2,426 SimPoints per world.
There are also Gold Editions available for some of the worlds, which include premium content. Gold Editions cost more SimPoints. You will also need to log in the website to download anything.
If you are looking for new content for your Sims 3 game, I suggest getting a new world.