-The world feels alive. The Strip alone has more life and fun in it than anything found in Fo3. Was anyone else completely disappointed by how dead and undeveloped Rivet City (the biggest settlement in the wasteland outside of the Brotherhood/Enclave) was? Yeah, good job fixing that with New Vegas, Obsidian. Big props.
-There are actual characters. With so few exceptions I could count them on my fingers*, there were no characters in Fallout 3. In this context when I say character I mean a character with a defined, reasonably believable personality that gets more than three or four lines of dialog before being shuffled off. I don't want Shakespeare level character depth, mind you, but almost the entire cast of Fo3 felt like a bunch of robots. Every follower I've tried in New Vegas has a personality, a story, a driving motivation that puts anything from Fo3 to shame, and the main and even secondary characters actually feel like people. Hell, even the robots feel more like people in this game than the characters in Fo3.
-Much, much better dialog. This cannot be overstated. Being forced to tell Three Dog "I want to find my daddy please!" in Fo3 was just ridiculous... and that conversation was one of the better ones in the game.
-Economy, agriculture, and industry are all present and accounted for. A world with an explanation for how the population continues to survive and grow, amazing! Obsidian actually took the time to explain how the city gets food and electricity, and for this I give them more major props.
-Gambling is awesome. Granted, luck makes it just an easy way to get over 100,000 caps, but it's still a blast and a huge advantage over Fo3. Though I would have appreciated some dice games (High roller suite? What did I high roll? There are NO DICE!!!), maybe they'll add that in DLC.
-Moral ambiguity, god I love it. It sure is nice to actually have to give a moment of thought to what the right thing to do in a situation is, instead of having the straight good and bwah-ha-ha ebil choices. No spoilers, but every faction is something that feels human and believable. Every faction has motives for what they do and a philosophy to justify it. And I like that even if you take the 'good' side, some of the missions still (might) make you feel a bit dirty.
-No more bobbleheads. Some might love them, but I found them to be an extremely annoying immersion breaker in what is supposed to be an rpg and a shooter. Bobbleheads don't belong in either genre. Good riddance. Implants do the same thing, but are actually believable within the setting.
*Braun, your dad, President Eden, Elder Lyons and Three Dog being the only exceptions that come to mind. Maybe Amata, too.
Edit: Bumping the thread to add refutations of more arguments that have cropped up to justify the ridiculous claim that Fallout 3 is better than New Vegas. Seemed like a better idea than creating a whole new thread.
First, I've seen the claim that Fallout 3 had a lot more to explore... yeah, no it didn't. It had fewer locations. I think what you're looking for, if you accept that argument, is a vast empty space full of cut-and-paste dungeons filled with the same things you've already seen twenty times. Is that seriously where a game developer should be devoting resources, towards making a huge, vast, empty world full of nothing to do but wander aimlessly shooting mutants, raiders, and the occasional Enclave trooper? I want a large world, too, but not at the cost of it being devoid of any serious content.
Second, I saw a couple people actually saying the Fallout 3 story is better, with a more personal story and more drive to continue turning the page to find out what happens next. I don't even know how to begin replying to something like that. I feel like I'm being asked to explain why Transformers 2 isn't a good film... if you don't know already, there just isn't much I can do to explain why that just plain isn't the case.
This isn't just a matter of opinion, as so many people like to claim when trying to defend something awful like the Star Wars prequels. A story can be objectively bad, and Fallout 3 had a story that was really, really poorly done. The writing lacked any subtlety, with a clearly defined good and a clearly defined evil, and neither side was remotely believable. I've mentioned this above, but it bears repeating because it's just such a glaring flaw. The plot holes and "...wtf?" moments in Fallout 3 are so numerous that I don't even think I should have to cite examples, but I will anyways:
-Why would Eden trust an enemy with the sole means of accomplishing his grand, evil master plan? Shouldn't a super computer be able to know immediately if the person is going to do as requested?
-How the hell do the kids in Little Lamplight have a population if they have no population growth? They kick people out before they are capable of (realistic) reproduction, they wouldn't last more than one generation.
-Why didn't the Enclave just neutralize the Brotherhood with an orbital strike the second they had them on the ropes? Seriously epic fail here. I would applaud this choice if they explained, for example, that the remaining Enclave wanted a ceasefire and didn't want to wantonly destroy another major faction.. but sadly, that isn't the case. Bethesda can't give the Enclave any humanity, that would be outrageous! So apparently the Enclave, in addition to mustache twirling ebilness, has a severe case of lethal stupidity.