Fallout 3 was a big mistake because it was Bethesda taking a loved series and turning it into whatever the hell they wanted. The original Fallout games were about humanity rebuilding civilisation after they destroyed it because they got greedy and couldn't get along. The Fallout games delved into human nature and how we would react to living in a life where civilisation was gone and now a new one was going to be born out of the ashes of the old world.
Fallout 1 and 2 (to a much lesser extent) both included these aspects. In Fallout 1 the Master was aware that people had caused their own downfall and that as humans we could not properly take care of ourselves, therefore he saw Super Mutants, which he could create, as the next step in human evolution. He wanted to create an army of mutants that would unify the human race and make all of their goals the same so we could live together and be more equal. However, there were many flaws such as people who had been living in the wasteland were not fit for transformation nad became dumb and brutish, whereas more "cleaner" humans became intelligent and strong beings. More imprtantly all female Super Mutants are sterile, which means that eventually the Super Mutant race will no longer be able to continue and they will die out.
Basically, Fallout 1 dealt with getting rid of human?s problems by getting rid of humans themselves.
Now let?s skip to Fallout 3, what was that about? Oh yeah some whiny kid whose Dad goes missing and now he wants to go find him. Then said dad wants to purify the water (which should've mostly cleared by now anyway) to give all the people of the East Coast fresh water, because apparently they don't know how to make wells or filter water like those of the West Coast.
Fallout 3 just didn't have the political or societal messages of the original Fallouts and instead sacrificed that for stupid quests that you only take part in to see what cool loot you will receive. Fallout 3 was filled with stupidity and nonsensical things, such as the Experimental MIRV and Mothership Zeta. Bethesda did away with rebuilding civilisation and decided they would focus more on petty survivors trying hard to create communities where they can survive the harshness of the apocalypse, 200 years after it has happened, made even more pathetic by the existence of the NCR in the West who by that time had made a large republic of more than 300,000 people with a fully working government and military.
Fallout: New Vegas thankfully went back to original messages of Fallout and continued civilisations rebirth with the NCR and Caesar's Legion, a force dedicated to fixing the mistakes of the old world by taking one of the most successful empires in history and getting rid of the negatives of it, mainly the senate and democracy.
Would an empire led by one man's ideals work in the long run? Most likely not.
Would the NCR, who can barely hold onto Hoover Dam and Vegas be able to continue to spread? Not at all if they continue to be lead by inept commanders.
Can Mr House control all of Vegas by himself, or will he eventually turn into the real life Howard Hughes and become obsessed with himself and eventually become cut off from the real? Of course, because it has already happened.
Independent New Vegas? People free to do what they? Yeah, not a chance.
Not to mention New Vegas felt much more like a role-playing game. In Fallout 3 you were forced to be someone who grew up in a Vault, you were told who your mother and father were, you had to be friends with Amata, Butch always hates you, you can?t join the Tunnel Snakes, all of the character have a set personality as to how they like or hate you, your age is pre-determined and you?re forced to love your dad and find him, also you?re forced to join the BoS and you can?t even join the Enclave, yeah sure you can put their FEV into the purifier but it doesn?t really change much. Also there just wasn?t enough choice in how you finish the main quest, you get two choices that barely differ from each other, you should?ve been able to destroy the purifier completely.
In New Vegas, however, you play a person who works for the Mojave Express, delivering a package and then unfortunately get shot in the head. That?s it, that?s all we know, that, and some guy called Ulysses apparently has some history with us and is now looking for the Courier.
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Just my two cents.