I won't get into how it's not a faithful game according to the lore right now, I think everyone knows this by now. I'll just point out why the game is bad on it's own two legs.
First, for one, it's way too easy for a RPG game. You can extend your character in practically every direction, there's no sense of specialization. Fallout 1 and 2 weren't that hard, but at least skill points weren't a banquet. At least you couldn't abuse the AI to a ridiculous degree that you can in Fallout 3. At least you couldn't launch nukes which is ridiculous in so many ways (that don't even have anything to do with it's gameplay implementation) that you could write a little essay on that alone.
Second, the storytelling is embarrassing. It lacks humor, it lacks wit, it's trying two things and failing at both:
1) It's trying to be funny, but it's way too far in the lolsorandumb territory. Humor in Fallout 1/2 worked because it was tempered with enough horror and because the writers were plain funny. Fallout 3 is somehow even goofier than Fallout 2, but more in a Cartoon Network sense with truly stupid shit like the anti-communism robot that could have perhaps even worked if it was actually funny, but it was just base.
2) It's trying to be immersive, but characters are far too often either plain good or plain evil. Brotherhood of Steel were compelling because you couldn't really figure out where exactly they stand -- the BoS offshoot in Fallout 3 are just saviors of the wasteland, full stop. Paladins in shining armor, out to get the mean Enclave. And the Enclave? They used to be menacing, looking down on the wasteland from above -- now they're easily defeated goofballs that don't even look the part. Their power armor in the first two games had them towering over any other human, now they're roughly the same height. Not that there's much they could have done to make them in the least bit scary since we already defeated them in Fallout 2 and we know exactly what they are. There's no awe or mystery. Sure, they shoehorned a tweest where it turns out they're controlled by a batshit AI, but I couldn't muster a fuck as they already felt shoehorned for the sake of "continuity" and fan service. Arbitrary plot twist that amounts to nothing doesn't help something like that. Then of course, there's laughable stuff like a children's cave colony, super mutants being nothing but dumb orcs (with an incredibly hamfisted excuse for why they're on the east coast in the first place), the existence of Megaton, the fact that your supermutant buddy feared a radioactive chamber despite him being immune to radiation -- this list can go on.
Third -- the world. It has no sense of space, it's a neat little amusement park with fun bite-sized adventure quests always in reach. Even if it knew how to look pretty, I never felt like I'm in a sprawling, terrifying wasteland, it felt little more than a sightseeing tour made for player's enjoyment.
It's not the worst game ever made, but it's very bad. And if you compare it to the first two games, it's completely pale in comparison.