Here's my attempt to summarize:
- In Fallout 3, there is huge freedom to choose how to respond to situations. The whole choose-your-own-adventure thing. Choice goes way beyond deciding whether to use a machine gun or a grenade (which is basically the only choice you get in the otherwise thrilling Call of Duty games).
- A big part of this choice is the ability to have personal politics: karma, dialogue options, a reputation, affiliation to different factions and people. You end up actually caring what happens and feeling responsible for the consequences of your actions. Far Cry 2 was brilliant, but it really played down this aspect of choice and missed out on being a masterpiece as a result.
- The setting is open world and that's great to explore. There are very few points where you get told "you can't come here yet, come back later".
- There is a huge amount of character development. All the stats and skills make a huge difference to how the game is played. Most action games just rely on your reflexes and the kit that you have.
- There is a realistic relationship with the physical world around you. Fallout has Physics. You can fall off buildings and drown. Mass Effect is a great game but it just doesn't have this. Also, Fallout 3 is very object oriented. You can carry what you like, within certain limits. You don't have a "heavy weapon" slot. Suppose you love your handguns and want to carry 5 different ones - go ahead. You find stuff just lying around as you would in real life without great big pink arrows pointing to them or messages saying "pick up the heavy machine gun and defend the fort". You can do novel things with the stuff lying around that isn't pre-scripted. Bodies don't fade away moments after death. And this is important: if you leave something somewhere you will find it later on, even years later.
- You can 'fail' a quest and the game continues. Contrast that with e.g. Grand Theft Auto San Andreas where you have to keep repeating some quest until you succeed. You somehow go back in time and end up having the same conversation (or car chase) with someone 20 times. It's so unreaslitic. In Fallout 3 you never have to repeat something unless you get killed.
- The setting is a top notch work of fantasy. Ridiculous, comical, exciting, intense, scary...not at all boring, but not too far from actual reality. You can still relate to it.
I'd like to hear your own thoughts. What is it that makes this game extra special?