Avatar is the new Wizard of Oz. It won't be long before all movies are in 3D. It's not an earth shattering difference - but it is an added layer within the experience that is going to the movies (and also helps justify the ticket price).
The movie itself is a good story that moves quickly enough that you don't notice the nearly 3 hours you will spend in your chair. It would be worth seeing in even in 2D.