The Fallout games were a marvel. They worked along a set of basic and important principles and remained true to them, which Bethesda also aspires to do. Straight from the official Fallout 3 website:
Mega levels of violence.
There is often no right solution.
There will always be multiple solutions.
The players actions affect the world.
There is a sense of urgency.
It's open ended.
The player will have a goal.
The player has control of his actions.
Simple Interface.
Speech will be lip-synched with the animation.
A wide variety of weapons and actions.
Detailed character creation rules.
The [development] Team is Motivated.
These are from the Fallout Vision Statement Interplay devised before actual work on Fallout began. All of these are principles at the heart of every Fallout game. They were what made Fallout innovative, fresh, and unique. Many games have attempted to recreate the atmosphere and adopt several of these principles (many of which have become cliche since then), but none have effectively captured what makes Fallout so great. How many games actually allow those with high Charisma to talk their way through important situations, even including the final boss? When they say there will
always be multiple solutions, they mean it.
Fallout 3 is the first true Fallout sequel in over half a decade. Since Black Isle studios was disbanded, many feared they would never see another Fallout game. And now a sequel has been made by the excellent developers of Oblivion, who haven't proven their open-world RPG chops again and again with the Elder Scrolls series. Fallout 3 is in extremely capable hands, and it is evident that Bethesda has remained faithful to the Fallout universe while adding their own flair to it. For me, this is just about the most anticipated game of a lifetime.
Also, some websites that should help you understand:
http://fallout.bethsoft.com/eng/home/home.php?fbid=Za0GN http://www.prepareforthefuture.com