Well, they're changing a lot more than you think. The dialog system has been reworked completely, the plot is completely new, and its a very good one too, which relates to the first two games. Theres a reputation system now and multiple main quests. They've actually improved the textures on the graphics a lot since the early gameplay videos, it looks BETTER than fallout 3, if only a little. Gameplay has been improved upon too, fixing a lot of F3's mistakes.
Really, the only thing that will be the same is the graphics and basic gameplay. But, alas that means A LOT to gamers these days, more than it should.
You can't judge a book by its cover, it will be a lot more different than fallout 3 than you would first think.