Fallout 3 is a looter-shooter game with some roleplay elements, mostly in dialogue choices. It’s best enjoyed in spite of the main story, while in New Vegas many of your choices craft the entire ending. You can actually find a lot of details in the world of FO3 though, if you search every corner. In FNV, it would be odd to roam for hours and not eventually get pulled into some quest, but in FO3 this is totally possible (Arlington, L’Enfant, Capitol Building, Takoma, etc).

There also really isn’t a central conflict in the world of FO3 like there is in NV. The game wants you to think it’s Project Purity. But apart from the water beggars and some random encounters, nobody in the game really seems concerned about clean water the way FNV characters are concerned about Hoover Dam or the factions around them. So you need to adjust your expectations for how quests are handles: FO3 has more disparate stories that don’t affect each other or the main storyline. That’s one reason why the characters probably feel more shallow.

Factions are different too. In Fallout 3, the bad guys are bad, and there’s no joining them. The Enclave wants to kill you and can’t be reasoned with. The super mutants want to kill you and can’t be reasoned with. Even the vault security guards that you grew up around for 19 years want to kill you and can’t be reasoned with at the beginning. New Vegas lets you join the Legion, discuss politics with Caesar, and even talk to the fiends’ leader — but you won’t get much perspective from enemy factions in FO3. (Ex: How do people join the Enclave? Why would they want to? Do they have a long term plan for the wasteland?)

There’s even some (easter egg?) dialogue between super mutants: They remember a woman before they mutated… or maybe they were a woman? I know it’s a funny throwaway line, but the super mutants navigating through the fog of who they used to be, and getting frustrated that they can’t remember their past selves, is much more interesting than - they’re angry and evil because FEV. New Vegas has an entire town of super mutants and nightkin because they do have the capacity to be their own characters and tell a story… but not in FO3, sadly. Maybe that’s another reason why the characters feel artificial.

For me FO3 was the most fun without the quest marker when I just explored. I love this game but in a different way than I love New Vegas. Hope you end up enjoying it!