The Shack's preview covers a range of topics, but it contains a good number of details about the differences between The Elder Scrolls IV: Oblivion and Fallout 3. For instance, the new Fallout is on a smaller scale than Oblivion, and players themselves "cannot simply 'do everything'." Also gone is Oblivion's relative leveling system, and Bethesda has reduced the number of in-game characters while enlisting additional voice actors to make interactions more varied. In addition, Bethesda has worked up a better third-person camera mode that features an over-the-shoulder perspective (although there's still a first-person mode).
Judging by Shacknews' preview, the people at Bethesda have been fans of the Fallout series from the beginning, and they're working hard to retain the tone of the original game.