its as simple as one word
choice
there is no other open world game that gives you more choice and options in terms of decision making than Fallout 3 (and New Vegas). obviously the main storyline for the most part is linear, but the 100 or so other quests in the games almost always allow you to make SOME choice
whether its choosing to help one person over another, ignoring said mission, killing those who ask for your help, etc., there's so much choice
you can do practically whatever you want in the game whenever you want.
there's a big world to explore, there's a quirky humorous retro art design and dialogue humor that a lot of people enjoy.
also to state that Fallout 3's graphics were bad for 2008 is ludicrous, you couldn't name many other games in that period that had such excellent visuals and also had to maintain anywhere near the amount of assets in a big game world at one time
there are tons of great things about the game
I think the reality is that if you fail to find practically anything good in Fallout 3 (or New Vegas for that matter) then its plainly obvious you're not a fan of a certain part of the RPG genre. if you can't get yourself into your imagination of character then I can see how it might be hard to get absorbed into a Fallout game as so much is up to the player to decide and a described character/story isn't always mouthfed
